Avoid collecting meaningless ScopedBlockingCall trace events

The call to base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope and
to WaitableEvent::wait(...) are generating trace events that are not
useful after the thread_id is set. Since it's only done at thread
creation, it cannot blocked otherwise and there is no point to get
a ScopedBlockingCall trace events.

R=gab@chromium.org

Change-Id: I1fdaa6b6d3e445fff00c850a1e41a35f95959e1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260939
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781694}

base/threading/thread.cc

@@ -278,9 +278,11 @@ void Thread::DetachFromSequence() {
 }
 
 PlatformThreadId Thread::GetThreadId() const {
-  // If the thread is created but not started yet, wait for |id_| being ready.
-  base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope allow_wait;
-  id_event_.Wait();
+  if (!id_event_.IsSignaled()) {
+    // If the thread is created but not started yet, wait for |id_| being ready.
+    base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope allow_wait;
+    id_event_.Wait();
+  }
   return id_;
 }