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 40741461320fa0affe36e8f7d8628d61691b1271..7f020072f330c755bf9af933cf9ae807fa07397c 100644
--- a/cc/trees/damage_tracker.cc
+++ b/cc/trees/damage_tracker.cc
@@ -287,6 +287,10 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
       layer->draw_transform(), gfx::Rect(layer->content_bounds()));
   data.Update(rect_in_target_space, mailboxId_);
 
+  // Union layer's update_rect with damage_rect

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

nit: missing period at the end of the line.

+  gfx::RectF unified_update_and_damage_rect =

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

The comment and variable names below are no longer properly named. I would just
call this damage_rect, make it clear in the comment above that this is the union
of update_/content_damage_rect and make the comment and variable names below
consistent with that.

+      gfx::UnionRects(layer->update_rect(), layer->damage_content_rect());
+
   if (layer_is_new || layer->LayerPropertyChanged()) {
     // If a layer is new or has changed, then its entire layer rect affects the
     // target surface.
@@ -295,11 +299,11 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
     // The layer's old region is now exposed on the target surface, too.
     // Note old_rect_in_target_space is already in target space.
     target_damage_rect->Union(old_rect_in_target_space);
-  } else if (!layer->update_rect().IsEmpty()) {
+  } else if (!unified_update_and_damage_rect.IsEmpty()) {
     // If the layer properties haven't changed, then the the target surface is
     // only affected by the layer's update area, which could be empty.
     gfx::Rect update_content_rect =
-        layer->LayerRectToContentRect(layer->update_rect());
+        layer->LayerRectToContentRect(unified_update_and_damage_rect);
     gfx::Rect update_rect_in_target_space = MathUtil::MapEnclosingClippedRect(
         layer->draw_transform(), update_content_rect);
     target_damage_rect->Union(update_rect_in_target_space);