Bug 1435022 - Minor cleanup on the Destroy function. r=sotaro

MozReview-Commit-ID: EnE6JKKmhgy

--HG--
extra : rebase_source : 49e11327c77968e649ae28bee7e5a72bc2a6f98a

gfx/layers/ipc/CompositorVsyncScheduler.cpp

@@ -106,11 +106,12 @@ CompositorVsyncScheduler::~CompositorVsyncScheduler()
 void
 CompositorVsyncScheduler::Destroy()
 {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+
   if (!mVsyncObserver) {
     // Destroy was already called on this object.
     return;
   }
-  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
   UnobserveVsync();
   mVsyncObserver->Destroy();
   mVsyncObserver = nullptr;

gfx/layers/ipc/CompositorVsyncScheduler.h

@@ -51,7 +51,11 @@ public:
   bool NotifyVsync(TimeStamp aVsyncTimestamp);
   void SetNeedsComposite();
 
+  /**
+   * Do cleanup. This must be called on the compositor thread.
+   */
   void Destroy();
+
   void ScheduleComposition();
   void CancelCurrentCompositeTask();
   bool NeedsComposite();