Bug 1392574 - Don't attempt to composite a titlebar image if we don't have any. r=mattwoodrow

MozReview-Commit-ID: 59NKegwszzO

--HG--
extra : rebase_source : d4a184681ba967ecf9c56f025a012729b06e0145

widget/cocoa/nsChildView.mm

@@ -2421,7 +2421,7 @@ void
 nsChildView::MaybeDrawTitlebar(GLManager* aManager)
 {
   MutexAutoLock lock(mEffectsLock);
-  if (!mIsCoveringTitlebar || mIsFullscreen) {
+  if (!mIsCoveringTitlebar || mIsFullscreen || mTitlebarRect.IsEmpty()) {
     return;
   }