Bug 1448485 - Remove assert in APZCallbackHelper and add comment about case it was triggered by. r=kats

MozReview-Commit-ID: IbXNISatv3Q

--HG--
extra : rebase_source : d88637454e58be5c18a1fa1dc0588a04eddc274c

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -675,15 +675,9 @@ PrepareForSetTargetAPZCNotification(nsIWidget* aWidget,
   }
 
   if (!scrollAncestor) {
-    MOZ_ASSERT(false);  // If you hit this, please file a bug with STR.
-
-    // Attempt some sort of graceful handling based on a theory as to why we
-    // reach this point...
-    // If we get here, the document element is non-null, valid, but doesn't have
-    // a displayport. It's possible that the init code in ChromeProcessController
-    // failed for some reason, or the document element got swapped out at some
-    // later time. In this case let's try to set a displayport on the document
-    // element again and bail out on this operation.
+    // This can happen if the document element gets swapped out after ChromeProcessController
+    // runs InitializeRootDisplayport. In this case let's try to set a displayport again and
+    // bail out on this operation.
     APZCCH_LOG("Widget %p's document element %p didn't have a displayport\n",
         aWidget, dpElement.get());
     APZCCallbackHelper::InitializeRootDisplayport(aRootFrame->PresShell());