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

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index f4d618b6c03f6ee143de2650d0c07fd039b0f90e..3c50683175da2d686eb493e432884c0d144ff1fc 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -707,6 +707,7 @@ void LayerImpl::ResetAllChangeTrackingForSubtree() {
   layer_property_changed_ = false;
 
   update_rect_ = gfx::RectF();
+  damage_rect_ = gfx::RectF();
 
   if (draw_properties_.render_surface)
     draw_properties_.render_surface->ResetPropertyChangedFlag();
@@ -1036,6 +1037,10 @@ void LayerImpl::SetUpdateRect(const gfx::RectF& update_rect) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::AddDamageRect(const gfx::RectF& damage_rect) {
+  damage_rect_ = gfx::UnionRects(damage_rect_, damage_rect);
+}
+
 void LayerImpl::SetContentBounds(const gfx::Size& content_bounds) {
   if (this->content_bounds() == content_bounds)
     return;