Adding more instrumentations to find jankiness.

Prior instrumentations showed that most of jank happens in:
* "StartReading(false);  // Read the first chunk." line in ResourceLoader::OnResponseStarted: 17.6 jph
* URLFetcherCore::OnReadCompleted: 6.15 jph
but outside of other instrumentations.

Instrumenting their not yet instrumented internals.

BUG=423948

Review URL: https://codereview.chromium.org/774513002

Cr-Commit-Position: refs/heads/master@{#306343}

content/browser/loader/resource_loader.cc

@@ -643,6 +643,10 @@ void ResourceLoader::CompleteResponseStarted() {
 }
 
 void ResourceLoader::StartReading(bool is_continuation) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION("423948 ResourceLoader::StartReading"));
+
   int bytes_read = 0;
   ReadMore(&bytes_read);
 
@@ -680,6 +684,10 @@ void ResourceLoader::ResumeReading() {
 }
 
 void ResourceLoader::ReadMore(int* bytes_read) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile1(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION("423948 ResourceLoader::ReadMore1"));
+
   DCHECK(!is_deferred());
 
   // Make sure we track the buffer in at least one place.  This ensures it gets
@@ -689,8 +697,8 @@ void ResourceLoader::ReadMore(int* bytes_read) {
   int buf_size;
   {
     // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
-    tracked_objects::ScopedTracker tracking_profile(
-        FROM_HERE_WITH_EXPLICIT_FUNCTION("423948 ResourceLoader::ReadMore"));
+    tracked_objects::ScopedTracker tracking_profile2(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION("423948 ResourceLoader::ReadMore2"));
 
     if (!handler_->OnWillRead(&buf, &buf_size, -1)) {
       Cancel();

net/url_request/url_fetcher_core.cc

@@ -436,16 +436,36 @@ void URLFetcherCore::OnReadCompleted(URLRequest* request,
   URLRequestThrottlerManager* throttler_manager =
       request->context()->throttler_manager();
   if (throttler_manager) {
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile1(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted1"));
+
     url_throttler_entry_ = throttler_manager->RegisterRequestUrl(url_);
   }
 
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile2(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLFetcherCore::OnReadCompleted2"));
+
   do {
     if (!request_->status().is_success() || bytes_read <= 0)
       break;
 
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile3(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted3"));
+
     current_response_bytes_ += bytes_read;
     InformDelegateDownloadProgress();
 
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile4(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted4"));
+
     const int result =
         WriteBuffer(new DrainableIOBuffer(buffer_.get(), bytes_read));
     if (result < 0) {
@@ -456,17 +476,38 @@ void URLFetcherCore::OnReadCompleted(URLRequest* request,
 
   const URLRequestStatus status = request_->status();
 
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile5(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLFetcherCore::OnReadCompleted5"));
+
   if (status.is_success())
     request_->GetResponseCookies(&cookies_);
 
   // See comments re: HEAD requests in ReadResponse().
   if (!status.is_io_pending() || request_type_ == URLFetcher::HEAD) {
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile6(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted6"));
+
     status_ = status;
     ReleaseRequest();
 
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile7(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted7"));
+
     // No more data to write.
     const int result = response_writer_->Finish(
         base::Bind(&URLFetcherCore::DidFinishWriting, this));
+
+    // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+    tracked_objects::ScopedTracker tracking_profile8(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "423948 URLFetcherCore::OnReadCompleted8"));
+
     if (result != ERR_IO_PENDING)
       DidFinishWriting(result);
   }