Worker: Don't run micro tasks when V8 Isolate is terminated

We should not run micro tasks when V8 Isolate is terminated because that causes
a crash.

This CL doesn't add tests because this is a timing issue and it's difficult to
reproduce it without complicating tests.

Bug: 775224
Change-Id: I1bb36050d4aa7fd6b9aefca1944eb63ee7e5f899
Reviewed-on: https://chromium-review.googlesource.com/734562
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511319}

third_party/WebKit/Source/core/workers/WorkerThread.cpp

@@ -189,19 +189,22 @@ void WorkerThread::WillProcessTask() {
 
 void WorkerThread::DidProcessTask() {
   DCHECK(IsCurrentThread());
-  Microtask::PerformCheckpoint(GetIsolate());
-  GlobalScope()->ScriptController()->GetRejectedPromises()->ProcessQueue();
   if (GlobalScope()->IsClosing()) {
     // This WorkerThread will eventually be requested to terminate.
     GetWorkerReportingProxy().DidCloseWorkerGlobalScope();
 
     // Stop further worker tasks to run after this point.
     PrepareForShutdownOnWorkerThread();
-  } else if (IsForciblyTerminated()) {
+    return;
+  }
+  if (IsForciblyTerminated()) {
     // The script has been terminated forcibly, which means we need to
     // ask objects in the thread to stop working as soon as possible.
     PrepareForShutdownOnWorkerThread();
+    return;
   }
+  Microtask::PerformCheckpoint(GetIsolate());
+  GlobalScope()->ScriptController()->GetRejectedPromises()->ProcessQueue();
 }
 
 v8::Isolate* WorkerThread::GetIsolate() {