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..3953ab3c36dae7a3378cc727503ced6af024426f 100644
--- a/cc/trees/damage_tracker.cc
+++ b/cc/trees/damage_tracker.cc
@@ -289,6 +289,16 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
       gfx::RectF(gfx::PointF(), layer->content_bounds()));
   data.Update(rect_in_target_space, mailboxId_);
 
+  // Union layer's update_rect with damage_rect
+  if (!layer->damage_rect().IsEmpty()) {
+    gfx::RectF rect = layer->update_rect();
+    gfx::RectF layer_rect = gfx::ScaleRect(layer->damage_rect(),
+                                           1 / layer->contents_scale_x(),
+                                           1 / layer->contents_scale_y());
+    layer->SetUpdateRect(gfx::UnionRects(rect, layer_rect));

[danakj, 2014/04/14 17:17:07]

instead of setting the UpdateRect on the layer, just do the union of the damage
rect and the update rect here locally. Changing the UpdateRect here will mess
with later debug visualizations that collect them.

[sohanjg, 2014/04/15 10:03:48]

Done.

+    layer->ResetDamageRect();

[danakj, 2014/04/14 17:17:07]

Instead of clearing the damage rect here, you can clear it in
LayerImpl::ResetAllChangeTrackingForSubtree.

[sohanjg, 2014/04/15 10:03:48]

Done.

+  }
+
   if (layer_is_new || layer->LayerPropertyChanged()) {
     // If a layer is new or has changed, then its entire layer rect affects the
     // target surface.