[wrapper-tracing] Don't try to schedule cleanup without a thread

Bug: chromium:763687
Change-Id: I06180bbdf66d837a6bafe800ce674cb8aaaf0ee9
Reviewed-on: https://chromium-review.googlesource.com/690155Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505314}

third_party/WebKit/Source/platform/bindings/ScriptWrappableVisitor.cpp

@@ -96,8 +96,10 @@ void ScriptWrappableVisitor::PerformCleanup() {
 }
 
 void ScriptWrappableVisitor::ScheduleIdleLazyCleanup() {
-  // Some threads (e.g. PPAPI thread) don't have a scheduler.
-  if (!Platform::Current()->CurrentThread()->Scheduler())
+  WebThread* const thread = Platform::Current()->CurrentThread();
+  // Thread might already be gone, or some threads (e.g. PPAPI) don't have a
+  // scheduler.
+  if (!thread || !thread->Scheduler())
     return;
 
   if (idle_cleanup_task_scheduled_)