ServiceWorker: Add null check of ExecutionContext in skipWaiting.

Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp

Index: Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp b/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
index f9b3dd25899fe75806e37b5b7494db94ccf5e639..28e66474bd2ed3a45aa200217331a82e28ace882 100644
--- a/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
@@ -126,10 +126,13 @@ void ServiceWorkerGlobalScope::close(ExceptionState& exceptionState)
 
 ScriptPromise ServiceWorkerGlobalScope::skipWaiting(ScriptState* scriptState)
 {
+    ExecutionContext* executionContext = scriptState->executionContext();
+    if (!executionContext)
+        return ScriptPromise();

[falken, 2015/01/08 04:31:52]

I have some questions.

1. What happens if TerminateExecution is called after this, is still safe to use
this executionContext variable? +haraken@ mentioned in
https://codereview.chromium.org/535193002/ crashes could still occur in that
scenario.
2. Why is the TerminateExecution getting called in the first place? We schedule
stopping the worker 30 seconds after it's started, it shouldn't happen during
this layout test.

[xiang, 2015/01/08 08:16:13]

I'm not sure about it in this case, from my code reading, it should be OK.
Anyway I added a "FIXME" comment.
I think the unregister() will trigger worker stop. 2 of 3 tests in this file
will unregister() without waiting the worker "complete" message.

[falken, 2015/01/08 08:28:05]

I see! That looks right.

+
     RefPtrWillBeRawPtr<ScriptPromiseResolver> resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = resolver->promise();
 
-    ExecutionContext* executionContext = scriptState->executionContext();
     ServiceWorkerGlobalScopeClient::from(executionContext)->skipWaiting(new SkipWaitingCallback(resolver));
     return promise;
 }