Bug 1290338. Part 2 - don't call back to owner functions if the watcher is destroyed. r=bholley

MozReview-Commit-ID: FQkJYtLTBNQ

--HG--
extra : rebase_source : c947ed2de25426815f5260be9b550dfa99f88950

xpcom/threads/StateWatching.h

@@ -271,7 +271,9 @@ private:
       MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
       MOZ_ASSERT(mStrongRef);
       RefPtr<OwnerType> ref = mStrongRef.forget();
-      ((*ref).*mCallbackMethod)();
+      if (!mDestroyed) {
+        ((*ref).*mCallbackMethod)();
+      }
     }
 
     OwnerType* mOwner; // Never null.