Revert of [Tracing] Create TracingCategoryObserver in gin/. (patchset c…

…hromium#2 id:20001 of https://codereview.chromium.org/2456073004/ )

Reason for revert:
Revert this patch because clusterfuzz complains about bad casting.

Original issue's description:
> [Tracing] Create TracingCategoryObserver in gin/.
>
> Previously we implemented TracingCategoryObserver in V8 in order to do fast
> category check when we enable V8 specific tracing feature in a unified way.
> This patch calls the the creation of TracingCategoryObserver from chrome and
> adds it to observer list.
>
> BUG=v8:5590
>
> Committed: https://crrev.com/f3b25db89a2dfaad59a90573fe355bc60db0a899
> Cr-Commit-Position: refs/heads/master@{#428756}

TBR=jochen@chromium.org,fmeawad@chromium.org,alph@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5590

Review-Url: https://codereview.chromium.org/2471563003
Cr-Commit-Position: refs/heads/master@{#429160}

gin/v8_platform.cc

@@ -10,7 +10,6 @@
 #include "base/threading/worker_pool.h"
 #include "base/trace_event/trace_event.h"
 #include "gin/per_isolate_data.h"
-#include "v8/include/v8-tracing.h"
 
 namespace gin {
 
@@ -155,11 +154,7 @@ namespace {
 class EnabledStateObserverImpl final
     : public base::trace_event::TraceLog::EnabledStateObserver {
  public:
-  EnabledStateObserverImpl() {
-    tracing_category_observer_ = v8::tracing::TracingCategoryObserver::Create();
-    observers_.insert(reinterpret_cast<v8::Platform::TraceStateObserver*>(
-        tracing_category_observer_.get()));
-  }
+  EnabledStateObserverImpl() = default;
 
   void OnTraceLogEnabled() final {
     base::AutoLock lock(mutex_);
@@ -197,8 +192,6 @@ class EnabledStateObserverImpl final
  private:
   base::Lock mutex_;
   std::unordered_set<v8::Platform::TraceStateObserver*> observers_;
-  std::unique_ptr<v8::tracing::TracingCategoryObserver>
-      tracing_category_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(EnabledStateObserverImpl);
 };