Mark as needing a compositing update when allocating or clearing a

PaintLayerScrollableArea.

Bug: 748955
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I838699c551ff56ad287e5f1759196c17cf8b698e
Reviewed-on: https://chromium-review.googlesource.com/597016Reviewed-by: Walter Korman <wkorman@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491455}

third_party/WebKit/Source/core/paint/PaintLayer.cpp

@@ -1589,9 +1589,11 @@ bool PaintLayer::RequiresScrollableArea() const {
 void PaintLayer::UpdateScrollableArea() {
   if (RequiresScrollableArea() && !scrollable_area_) {
     scrollable_area_ = PaintLayerScrollableArea::Create(*this);
+    Compositor()->SetNeedsCompositingUpdate(kCompositingUpdateRebuildTree);
   } else if (!RequiresScrollableArea() && scrollable_area_) {
     scrollable_area_->Dispose();
     scrollable_area_.Clear();
+    Compositor()->SetNeedsCompositingUpdate(kCompositingUpdateRebuildTree);
   }
 }