Don't assert when ServiceWorker::from is passed an empty promise

Source/modules/serviceworkers/ServiceWorker.cpp

Index: Source/modules/serviceworkers/ServiceWorker.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorker.cpp b/Source/modules/serviceworkers/ServiceWorker.cpp
index b1de87b41bdaa829ed13beb2cea9383ad9f28c18..1ec57d91826e4bc951b59582724f8068f950e0a3 100644
--- a/Source/modules/serviceworkers/ServiceWorker.cpp
+++ b/Source/modules/serviceworkers/ServiceWorker.cpp
@@ -157,7 +157,8 @@ PassRefPtr<ServiceWorker> ServiceWorker::from(ScriptPromiseResolverWithContext*
 {
     RefPtr<ServiceWorker> serviceWorker = ServiceWorker::from(resolver->scriptState()->executionContext(), worker);
     ScriptState::Scope scope(resolver->scriptState());
-    serviceWorker->waitOnPromise(resolver->promise());
+    if (!resolver->promise().isEmpty())

[haraken, 2014/06/17 00:51:55]

Shall we move this check into ServiceWorker::waitOnPromise so that we don't need
to add the same check for all call sites (although we currently don't have any
other call site of waitOnPromise)?

[falken, 2014/06/17 01:07:51]

Good idea, I've done this.

+        serviceWorker->waitOnPromise(resolver->promise());
     return serviceWorker;
 }