cc: Support animating scroll offset

cc/animation/layer_animation_value_provider.h

Index: cc/animation/layer_animation_value_provider.h
diff --git a/cc/animation/layer_animation_value_provider.h b/cc/animation/layer_animation_value_provider.h
new file mode 100644
index 0000000000000000000000000000000000000000..de101d8d115206444ee821add0c42eb01130e06a
--- /dev/null
+++ b/cc/animation/layer_animation_value_provider.h
@@ -0,0 +1,22 @@
+// 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 {
+

[Ian Vollick, 2013/11/29 15:29:29]

A comment explaining the need for this would be helpful. IIRC, it's because
scroll animations (unlike the other types of animation) need to start from the
'current' value. This value needs to be provided somehow, and the
LayerAnimationValueProvider is our means of doing so. That's right, right?

[ajuma, 2013/11/29 21:18:41]

Comment added. Note that I've avoided mentioning "scroll" specifically in the
comment, since it's reasonable to do the same thing for other properties (e.g.
this approach gives a way to interrupt CSS transitions without jitter).

+class CC_EXPORT LayerAnimationValueProvider {
+ public:
+  virtual ~LayerAnimationValueProvider() {}
+
+  virtual gfx::Vector2dF ScrollOffsetForAnimation() const = 0;
+};
+
+}  // namespace cc
+
+#endif  // CC_ANIMATION_LAYER_ANIMATION_VALUE_PROVIDER_H_