Bug 1289650 - Ensure the APZEventState object is destroyed on the main thread. r=kats

MozReview-Commit-ID: Aeq5CA22VLr

gfx/layers/apz/util/ChromeProcessController.cpp

@@ -88,6 +88,7 @@ ChromeProcessController::Destroy()
 
   MOZ_ASSERT(MessageLoop::current() == mUILoop);
   mWidget = nullptr;
+  mAPZEventState = nullptr;
 }
 
 nsIPresShell*