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..9c183f1d61a0c27713f20e0d22fd6e10a9b36dca 100644
--- a/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerGlobalScope.cpp
@@ -126,10 +126,14 @@ void ServiceWorkerGlobalScope::close(ExceptionState& exceptionState)
 
 ScriptPromise ServiceWorkerGlobalScope::skipWaiting(ScriptState* scriptState)
 {
+    ExecutionContext* executionContext = scriptState->executionContext();
+    // FIXME: short-term fix, see details at: https://codereview.chromium.org/535193002/.
+    if (!executionContext)
+        return ScriptPromise();
+
     RefPtrWillBeRawPtr<ScriptPromiseResolver> resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = resolver->promise();
 
-    ExecutionContext* executionContext = scriptState->executionContext();
     ServiceWorkerGlobalScopeClient::from(executionContext)->skipWaiting(new SkipWaitingCallback(resolver));
     return promise;
 }