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

cc/trees/layer_tree_impl.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_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/animation/keyframed_animation_curve.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/debug/traced_value.h"
@@ skipping 262 matching lines @@
       return;
 
   if (!root_layer())
     return;
 
   {
     TRACE_EVENT1("cc",
                  "LayerTreeImpl::UpdateDrawProperties",
                  "IsActive",
                  IsActiveTree());
+
+    // TODO(wjmaclean): when settings().use_pinch_virtual_viewport is specified,

[enne (OOO), 2013/06/12 19:44:38]

Maybe LayerTreeHost can be told the id of the page scale layer and this code can
be ignorant about settings and which layer that actually is.

[wjmaclean, 2013/06/12 19:54:36]

Yes, that is the ultimate goal. LTH will be told which layer ids to use for
inner/outer/page layers, and set that here. That's planned for the next CL.

+    // pass the page_scale_layer instead of root_scroll_layer_ for the page
+    // scale application layer in the following call.
     LayerTreeHostCommon::CalculateDrawProperties(
         root_layer(),
         layer_tree_host_impl_->DeviceViewport().size(),
         layer_tree_host_impl_->DeviceTransform(),
         device_scale_factor(),
         total_page_scale_factor(),
         root_scroll_layer_,
         MaxTextureSize(),
         settings().can_use_lcd_text,
         settings().layer_transforms_should_scale_layer_contents,
@@ skipping 274 matching lines @@
 
 void LayerTreeImpl::ClearLatencyInfo() {
   latency_info_.Clear();
 }
 
 void LayerTreeImpl::WillModifyTilePriorities() {
   layer_tree_host_impl_->SetNeedsManageTiles();
 }
 
 }  // namespace cc