[Jobs API]: Implement JobDelegate::IsJoiningThread.

To let the user do special handling on the main thread e.g. Scavenging
uses different tracing categories for background/foreground threads.

Change-Id: I001b669e318bf88f39358ddb96482ec5ed4b62a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418986
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809328}

base/task/post_job.h

@@ -57,6 +57,12 @@ class BASE_EXPORT JobDelegate {
   // reused by a different thread after a worker_task returns.
   uint8_t GetTaskId();
 
+  // Returns true if the current task is called from the thread currently
+  // running JobHandle::Join().
+  bool IsJoiningThread() const {
+    return pooled_task_runner_delegate_ == nullptr;
+  }
+
  private:
   static constexpr uint8_t kInvalidTaskId = std::numeric_limits<uint8_t>::max();
 

gin/v8_platform.cc

@@ -284,6 +284,7 @@ class JobDelegateImpl : public v8::JobDelegate {
     delegate_->NotifyConcurrencyIncrease();
   }
   uint8_t GetTaskId() override { return delegate_->GetTaskId(); }
+  bool IsJoiningThread() const override { return delegate_->IsJoiningThread(); }
 
  private:
   base::JobDelegate* delegate_;