cc: Add support for partial swaps when using impl-side painting.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index 9af9eb5aa590ce200d403272baa45ee9293e8b7a..5543b7515acb2952f8a04b708ffd5286333395e9 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -454,6 +454,10 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
 
   const gfx::RectF& update_rect() const { return update_rect_; }
 
+  void UnionDamageContentRect(gfx::RectF damage_content_rect);

[reveman, 2014/04/15 16:25:59]

nit: pass by const reference instead

[reveman, 2014/04/15 16:25:59]

I would prefer SetContentDamageRect() and have the caller responsible for union
of rects. The code in NotifyTileInitialized would look something like this:

layer_impl->SetContentDamageRect(
    gfx::UnionRects(layer_impl->content_damage_rect(),
                    tile_content_rect_in_layer_space))

[enne (OOO), 2014/04/15 17:27:27]

(Personally I think what you suggest is a violation of demeter, since it makes
TileManager responsible for the representation of damage on layers.  It exposes
that damage is kept as a RectF internally, rather than as a region or something.
 Not that we want to, but it's just API smell, in my opinion.  I preferred the
original version.)

[reveman, 2014/04/15 17:50:48]

Makes sense. I prefer AddContentDamageRect() in that case.

[enne (OOO), 2014/04/15 17:55:42]

How about AddLayerDamageRect?

Sorry to keep quibble, but it's awkward to keep content rects on
PictureLayerImpl because it modifies its own content scale during pinch zoom. 
So, whenever you changed it, you'd have to rescale the content damage rect. 
It's better to keep damage in layer space to avoid potentially forgetting to do
this.

+
+  const gfx::RectF& damage_content_rect() const { return damage_content_rect_; }

[reveman, 2014/04/15 16:25:59]

content_damage_rect()? makes it sound more like a getter.

+
   virtual base::DictionaryValue* LayerTreeAsJson() const;
 
   void SetStackingOrderChanged(bool stacking_order_changed);
@@ -652,6 +656,8 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
   // Uses layer (not content) space.
   gfx::RectF update_rect_;
 
+  gfx::RectF damage_content_rect_;
+
   // Manages animations for this layer.
   scoped_refptr<LayerAnimationController> layer_animation_controller_;