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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 072e8e7efbf2d6ae30186f8514e2d47b790961b8..90ac2f33806f04244949606caa64e97e3d25a0a2 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1195,7 +1195,14 @@ void LayerTreeHostImpl::DidModifyTilePriorities() {
 void LayerTreeHostImpl::DidInitializeVisibleTile() {
   // TODO(reveman): Determine tiles that changed and only damage
   // what's necessary.
-  SetFullRootLayerDamage();
+  gfx::Rect rect = tile_manager_->GetDamagedRect();
+  if (rect.width() * rect.height() <=
+      DrawViewportSize().width() * DrawViewportSize().height())
+    SetViewportDamage(rect);
+  else
+    SetFullRootLayerDamage();
+  tile_manager_->ResetDamagedRect();
+
   if (client_ && !client_->IsInsideDraw())
     client_->DidInitializeVisibleTileOnImplThread();
 }