cc: Support animating scroll offset

cc/animation/layer_animation_value_provider.h

+// Copyright 2013 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_ANIMATION_LAYER_ANIMATION_VALUE_PROVIDER_H_
+#define CC_ANIMATION_LAYER_ANIMATION_VALUE_PROVIDER_H_
+
+#include "cc/base/cc_export.h"
+#include "ui/gfx/vector2d_f.h"
+
+namespace cc {
+
+// A LayerAnimationValueProvider is used for determining the starting value
+// for animations that start at their 'current' value rather than at a
+// pre-specified value.
+class CC_EXPORT LayerAnimationValueProvider {
+ public:
+  virtual ~LayerAnimationValueProvider() {}
+
+  virtual gfx::Vector2dF ScrollOffsetForAnimation() const = 0;
+};
+
+}  // namespace cc
+
+#endif  // CC_ANIMATION_LAYER_ANIMATION_VALUE_PROVIDER_H_