Bug 1525450 - Guard against a null controller. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D19057

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -814,17 +814,16 @@ void APZCTreeManager::NotifyScrollbarDragInitiated(
     ScrollDirection aDirection) const {
   RefPtr<GeckoContentController> controller =
       GetContentController(aGuid.mLayersId);
-  MOZ_ASSERT(controller);
-  controller->NotifyAsyncScrollbarDragInitiated(aDragBlockId, aGuid.mScrollId,
-                                                aDirection);
+  if (controller) {
+    controller->NotifyAsyncScrollbarDragInitiated(aDragBlockId, aGuid.mScrollId,
+                                                  aDirection);
+  }
 }
 
 void APZCTreeManager::NotifyScrollbarDragRejected(
     const ScrollableLayerGuid& aGuid) const {
   RefPtr<GeckoContentController> controller =
       GetContentController(aGuid.mLayersId);
-  // If you hit this crash and have STR, please file a bug!
-  MOZ_ASSERT(controller);
   if (controller) {
     controller->NotifyAsyncScrollbarDragRejected(aGuid.mScrollId);
   }