[exp] cc: Introduce cc::CompositorMutator.

cc/trees/layer_tree_host_common.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_common.h"
 
 #include <algorithm>
 
 #include "base/trace_event/trace_event.h"
 #include "cc/base/math_util.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/layer_iterator.h"
 #include "cc/layers/render_surface.h"
 #include "cc/layers/render_surface_impl.h"
 #include "cc/trees/draw_property_utils.h"
 #include "cc/trees/layer_tree_host.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 #include "ui/gfx/geometry/vector2d_conversions.h"
 #include "ui/gfx/transform.h"
 #include "ui/gfx/transform_util.h"
 
 namespace cc {
 
+Mutation::Mutation() {
+}
+Mutation::~Mutation() {
+}
+
 ScrollAndScaleSet::ScrollAndScaleSet()
     : page_scale_delta(1.f), top_controls_delta(0.f) {
 }
 
 ScrollAndScaleSet::~ScrollAndScaleSet() {}
 
 template <typename LayerType>
 static gfx::Vector2dF GetEffectiveScrollDelta(LayerType* layer) {
   // Layer's scroll offset can have an integer part and fractional part.
   // Due to Blink's limitation, it only counter-scrolls the position-fixed
@@ skipping 2520 matching lines @@
       inputs->current_render_surface_layer_list_id);
 
   // The dummy layer list should not have been used.
   DCHECK_EQ(0u, dummy_layer_list.size());
   // A root layer render_surface should always exist after
   // CalculateDrawProperties.
   DCHECK(inputs->root_layer->render_surface());
 }
 
 }  // namespace cc