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 2faa6b2849543c5d7e0ed20a9e3b9a1f2674a5ab..55475fb0cc2c531f137da08a996e7d8eafd5d3f1 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1103,7 +1103,6 @@ DrawSwapReadbackResult::DrawResult LayerTreeHostImpl::PrepareToDraw(
   if (active_tree_->root_layer()) {
     device_viewport_damage_rect.Union(viewport_damage_rect_);
     viewport_damage_rect_ = gfx::Rect();
-
     active_tree_->root_layer()->render_surface()->damage_tracker()->
         AddDamageNextUpdate(device_viewport_damage_rect);
   }
@@ -1194,9 +1193,15 @@ void LayerTreeHostImpl::DidModifyTilePriorities() {
 }
 
 void LayerTreeHostImpl::DidInitializeVisibleTile() {
-  // TODO(reveman): Determine tiles that changed and only damage
-  // what's necessary.
-  SetFullRootLayerDamage();
+  std::map<int, gfx::Rect> map = tile_manager_->GetTileRectLayerMap();
+  for (std::map<int, gfx::Rect>::iterator it = map.begin(); it != map.end();
+       ++it) {
+    if (active_tree_)
+      active_tree_->FindActiveTreeLayerById(it->first)
+          ->SetInitializedTileRect(it->second);
+  }
+  tile_manager_->ResetTileRectLayerMap();
+
   if (client_ && !client_->IsInsideDraw())
     client_->DidInitializeVisibleTileOnImplThread();
 }