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

cc/trees/damage_tracker.cc

Index: cc/trees/damage_tracker.cc
diff --git a/cc/trees/damage_tracker.cc b/cc/trees/damage_tracker.cc
index 4d0e9353eff098a12f39f88da672c6ded7052b76..7072e14d98eec2e8e29f5c123d7208fabe552fed 100644
--- a/cc/trees/damage_tracker.cc
+++ b/cc/trees/damage_tracker.cc
@@ -306,6 +306,13 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
         MathUtil::MapClippedRect(layer->draw_transform(), update_content_rect);
     target_damage_rect->Union(update_rect_in_target_space);
   }
+
+  if (!layer->initialized_tile_rect().IsEmpty()) {
+    gfx::RectF update_tile_rect_in_screen_space = MathUtil::MapClippedRect(
+        layer->screen_space_transform(), layer->initialized_tile_rect());

[reveman, 2014/04/11 18:11:34]

I think we should unite this with update_rect() by not looking at update_rect
above and instead required that damage to be added to the damage rect used for
tile invalidation.

[sohanjg, 2014/04/14 10:16:11]

Done.
ptal, i am uniting with layers' update_rect before tracking the damage area, so
that the existing logic is not affected.

+    current_damage_rect_.Union(update_tile_rect_in_screen_space);
+    layer->SetInitializedTileRect(gfx::Rect());

[reveman, 2014/04/11 18:11:34]

I don't think this is the right place to reset the damage rect? what if this is
called twice?

[sohanjg, 2014/04/14 10:16:11]

this will be called only when we calculate renderpass, once for each
preparetodraw.
Or its better to reset it after frame is drawn (DrawFrame()) ?

+  }
 }
 
 void DamageTracker::ExtendDamageForRenderSurface(