Added experiment for changing viewport scroll ordering.

cc/layers/viewport.h

 // Copyright 2015 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.
 
 #ifndef CC_LAYERS_VIEWPORT_H_
 #define CC_LAYERS_VIEWPORT_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/layers/layer_impl.h"
 #include "ui/gfx/geometry/vector2d_f.h"
@@ skipping 10 matching lines @@
 class CC_EXPORT Viewport {
  public:
   struct ScrollResult {
     gfx::Vector2dF applied_delta;
     gfx::Vector2dF unused_scroll_delta;
     gfx::Vector2dF top_controls_applied_delta;
   };
 
   static scoped_ptr<Viewport> Create(LayerTreeHostImpl* host_impl);
 
+  // Differs from scrolling in that only the visual viewport is moved, without
+  // affecting the top controls or outer viewport.
+  void Pan(const gfx::Vector2dF& delta);
+
   // Scrolls the viewport, applying the unique bubbling between the inner and
   // outer viewport. Scrolls can be consumed by top controls.
   ScrollResult ScrollBy(const gfx::Vector2dF& delta,
                         const gfx::Point& viewport_point,
-                        bool is_wheel_scroll);
+                        bool is_wheel_scroll,
+                        bool affect_top_controls);
 
  private:
   explicit Viewport(LayerTreeHostImpl* host_impl);
 
   bool ShouldTopControlsConsumeScroll(const gfx::Vector2dF& scroll_delta) const;
   gfx::Vector2dF AdjustOverscroll(const gfx::Vector2dF& delta) const;
 
   // Sends the delta to the top controls, returns the amount applied.
   gfx::Vector2dF ScrollTopControls(const gfx::Vector2dF& delta);
 
   gfx::ScrollOffset MaxTotalScrollOffset() const;
   gfx::ScrollOffset TotalScrollOffset() const;
 
   LayerImpl* InnerScrollLayer() const;
   LayerImpl* OuterScrollLayer() const;
 
   LayerTreeHostImpl* host_impl_;
 
   DISALLOW_COPY_AND_ASSIGN(Viewport);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_VIEWPORT_H_