Bug 1455285 - In MoveScrollbarForLayerMargin(), move the actual scrollbar thumb layer, not the scrollbar container layer. r=kats

This was regressed by bug 1420512, which changed things so that
ScrollbarData::mDirection is set for both kinds of scrollbar layers.

MozReview-Commit-ID: 3UHFSOgDtWj

--HG--
extra : rebase_source : 25bc732e4216dbb1971bec57421e20698126f8f2

gfx/layers/composite/AsyncCompositionManager.cpp

@@ -758,7 +758,8 @@ MoveScrollbarForLayerMargin(Layer* aRoot, FrameMetrics::ViewID aRootScrollId,
   // adjustment on the layer tree.
   Layer* scrollbar = BreadthFirstSearch<ReverseIterator>(aRoot,
     [aRootScrollId](Layer* aNode) {
-      return (aNode->GetScrollbarData().mDirection.isSome() &&
+      return (aNode->GetScrollbarData().IsThumb() &&
+              aNode->GetScrollbarData().mDirection.isSome() &&
               *aNode->GetScrollbarData().mDirection == ScrollDirection::eHorizontal &&
               aNode->GetScrollbarData().mTargetViewId == aRootScrollId);
     });