trace_event: Don't initialize thread trace buffer during thread shutdown

If we try to record a trace event during MessageLoop shutdown, we can
get to a state where the MessageLoop is present but the thread task
runner handle has been reset. Since we require both in order to
initialize the thread local event buffer, this patch makes sure the
initialization is skipped if that condition isn't met.

Bug: 906795
Change-Id: I9ead620573387f8c329d954425ef43c60547bbef
Reviewed-on: https://chromium-review.googlesource.com/c/1343263Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609677}

base/trace_event/trace_log.cc

@@ -409,13 +409,15 @@ TraceLog::TraceLog()
 TraceLog::~TraceLog() = default;
 
 void TraceLog::InitializeThreadLocalEventBufferIfSupported() {
-  // A ThreadLocalEventBuffer needs the message loop
+  // A ThreadLocalEventBuffer needs the message loop with a task runner
   // - to know when the thread exits;
   // - to handle the final flush.
-  // For a thread without a message loop or the message loop may be blocked, the
-  // trace events will be added into the main buffer directly.
-  if (thread_blocks_message_loop_.Get() || !MessageLoopCurrent::IsSet())
+  // For a thread without a message loop or if the message loop may be blocked,
+  // the trace events will be added into the main buffer directly.
+  if (thread_blocks_message_loop_.Get() || !MessageLoopCurrent::IsSet() ||
+      !ThreadTaskRunnerHandle::IsSet()) {
     return;
+  }
   HEAP_PROFILER_SCOPED_IGNORE;
   auto* thread_local_event_buffer = thread_local_event_buffer_.Get();
   if (thread_local_event_buffer &&