Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

cc/trees/layer_tree_host.cc

 // Copyright 2011 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 <algorithm>
 #include <stack>
 
 #include "base/bind.h"
@@ skipping 700 matching lines @@
                                  ResourceUpdateQueue* queue) {
   TRACE_EVENT1("cc", "LayerTreeHost::UpdateLayers",
                "commit_number", commit_number());
 
   LayerList update_list;
   {
     UpdateHudLayer();
 
     Layer* root_scroll = FindFirstScrollableLayer(root_layer);
 
+    // TODO(wjmaclean): when settings().use_pinch_virtual_viewport is specified,
+    // pass the page_scale_layer instead of root scroll layer for the page
+    // scale application layer in the following call.
     TRACE_EVENT0("cc", "LayerTreeHost::UpdateLayers::CalcDrawProps");
     LayerTreeHostCommon::CalculateDrawProperties(
         root_layer,
         device_viewport_size(),
         gfx::Transform(),
         device_scale_factor_,
         page_scale_factor_,
         root_scroll,
         GetRendererCapabilities().max_texture_size,
         settings_.can_use_lcd_text,
@@ skipping 349 matching lines @@
     bool start_ready_animations = true;
     (*iter).second->UpdateState(start_ready_animations, NULL);
   }
 }
 
 skia::RefPtr<SkPicture> LayerTreeHost::CapturePicture() {
   return proxy_->CapturePicture();
 }
 
 }  // namespace cc