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 0e305b30b0cc28b7cddb4d610bcc7bef3be343f9..eb7ba60ba7f2a66d9788590983a5d69e161c175b 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -701,6 +701,7 @@ void LayerImpl::ResetAllChangeTrackingForSubtree() {
   layer_property_changed_ = false;
 
   update_rect_ = gfx::RectF();
+  damage_content_rect_ = gfx::RectF();
 
   if (draw_properties_.render_surface)
     draw_properties_.render_surface->ResetPropertyChangedFlag();
@@ -1017,6 +1018,11 @@ void LayerImpl::SetUpdateRect(const gfx::RectF& update_rect) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::UnionDamageContentRect(gfx::RectF damage_content_rect) {
+  damage_content_rect_ =
+      gfx::UnionRects(damage_content_rect_, damage_content_rect);
+}
+
 void LayerImpl::SetContentBounds(const gfx::Size& content_bounds) {
   if (this->content_bounds() == content_bounds)
     return;