Oilpan: Add TRACE_EVENT0 to ThreadState::waitUntilSweepersDone.

This event indicates that parallelized sweeping doesn't work well.

BUG=420515
TEST=None; No user-visible behavior changes.

Review URL: https://codereview.chromium.org/672033003

git-svn-id: svn://svn.chromium.org/blink/trunk@184234 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/heap/ThreadState.cpp

@@ -1029,6 +1029,7 @@ void ThreadState::unregisterSweepingTask()
 
 void ThreadState::waitUntilSweepersDone()
 {
+    TRACE_EVENT0("blink_gc", "ThreadState::waitUntilSweepersDone");
     MutexLocker locker(m_sweepMutex);
     while (m_numberOfSweeperTasks > 0)
         m_sweepThreadCondition.wait(m_sweepMutex);