Scrollbars should get notifications from all scrollable layers up to container.

cc/layers/scrollbar_layer_unittest.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/containers/hash_tables.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/layers/append_quads_data.h"
 #include "cc/layers/painted_scrollbar_layer.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
 #include "cc/layers/scrollbar_layer_interface.h"
@@ skipping 34 matching lines @@
   if (use_solid_color_scrollbar) {
     const bool kIsLeftSideVerticalScrollbar = false;
     child2 = SolidColorScrollbarLayer::Create(scrollbar->Orientation(),
                                               thumb_thickness,
                                               track_start,
                                               kIsLeftSideVerticalScrollbar,
                                               child1->id());
   } else {
     child2 = PaintedScrollbarLayer::Create(scrollbar.Pass(), child1->id());
   }
+  child2->ToScrollbarLayer()->SetClipLayer(layer_tree_root->id());
   layer_tree_root->AddChild(child1);
   layer_tree_root->InsertChild(child2, reverse_order ? 0 : 1);
   host->SetRootLayer(layer_tree_root);
   return host->CommitAndCreateLayerImplTree();
 }
 
 TEST(ScrollbarLayerTest, ResolveScrollLayerPointer) {
   scoped_ptr<FakeLayerTreeHost> host = FakeLayerTreeHost::Create();
   scoped_ptr<Scrollbar> scrollbar(new FakeScrollbar);
   LayerImpl* layer_impl_tree_root = LayerImplForScrollAreaAndScrollbar(
@@ skipping 818 matching lines @@
   TestScale(gfx::Rect(1240, 0, 15, 1333), 2.7754839f);
   TestScale(gfx::Rect(1240, 0, 15, 677), 2.46677136f);
 
   // Horizontal Scrollbars.
   TestScale(gfx::Rect(0, 1240, 1333, 15), 2.7754839f);
   TestScale(gfx::Rect(0, 1240, 677, 15), 2.46677136f);
 }
 
 }  // namespace
 }  // namespace cc