Trace blocking call in TaskQueueImpl

As discussed in https://crrev.com/c/1449653 add a trace to make it
easier to spot priority inversions happeingin due to this wait.

Bug: 927548
Change-Id: I7f96079c5b290eabadc4b127ee15ece52f57f356
Reviewed-on: https://chromium-review.googlesource.com/c/1454919
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632351}

base/task/sequence_manager/task_queue_impl.cc

@@ -14,6 +14,7 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "base/trace_event/blame_context.h"
+#include "base/trace_event/common/trace_event_common.h"
 #include "build/build_config.h"
 
 namespace base {
@@ -144,6 +145,7 @@ scoped_refptr<SingleThreadTaskRunner> TaskQueueImpl::CreateTaskRunner(
 }
 
 void TaskQueueImpl::UnregisterTaskQueue() {
+  TRACE_EVENT0("base", "TaskQueueImpl::UnregisterTaskQueue");
   // Detach task runners.
   {
     ScopedAllowBaseSyncPrimitivesOutsideBlockingScope allow_wait;