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 135a6163c0c9489e99387f7d0fbde13d2b2e8b4c..d7d4f2b835619322de183a5a0e950a7c5ac11867 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -1017,6 +1017,10 @@ void LayerImpl::SetUpdateRect(const gfx::RectF& update_rect) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::UnionDamageRect(gfx::Rect 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;