Refactor LayerTreeImpl's scrollbar map to be keyed on element ids

cc/trees/layer_tree_host_unittest_damage.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 "cc/trees/layer_tree_host.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/time/time.h"
@@ skipping 322 matching lines @@
 
 class LayerTreeHostScrollbarDamageTest : public LayerTreeHostDamageTest {
   void SetupTree() override {
     scoped_refptr<Layer> root_layer = Layer::Create();
     root_layer->SetBounds(gfx::Size(400, 400));
     root_layer->SetMasksToBounds(true);
     layer_tree_host()->SetRootLayer(root_layer);
 
     scoped_refptr<Layer> scroll_clip_layer = Layer::Create();
     content_layer_ = FakePictureLayer::Create(&client_);
+    content_layer_->SetElementId(
+        LayerIdToElementIdForTesting(content_layer_->id()));
     content_layer_->SetScrollClipLayerId(scroll_clip_layer->id());
     content_layer_->SetScrollOffset(gfx::ScrollOffset(10, 20));
     content_layer_->SetBounds(gfx::Size(100, 200));
     content_layer_->SetIsDrawable(true);
     scroll_clip_layer->SetBounds(
         gfx::Size(content_layer_->bounds().width() - 30,
                   content_layer_->bounds().height() - 50));
     scroll_clip_layer->AddChild(content_layer_);
     root_layer->AddChild(scroll_clip_layer);
 
@@ skipping 173 matching lines @@
 
   void AfterTest() override { EXPECT_EQ(3, num_draws_); }
 
   int num_draws_;
 };
 
 MULTI_THREAD_TEST_F(LayerTreeHostDamageTestScrollbarCommitDoesNoDamage);
 
 }  // namespace
 }  // namespace cc