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

cc/resources/tile_manager.cc

Index: cc/resources/tile_manager.cc
diff --git a/cc/resources/tile_manager.cc b/cc/resources/tile_manager.cc
index 8dc9c29ff0ae4527fc343c60bf58bf7b88a6092f..14f4f0da6acbc4afd5a5184c8ee0ba2e8361a627 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -462,6 +462,7 @@ TileManager::TileManager(
   };
   raster_worker_pool_delegate_ = RasterWorkerPoolDelegate::Create(
       this, raster_worker_pools, arraysize(raster_worker_pools));
+  damage_rect_ = gfx::Rect(0, 0, 0, 0);
 }
 
 TileManager::~TileManager() {
@@ -497,6 +498,10 @@ void TileManager::Release(Tile* tile) {
   released_tiles_.push_back(tile);
 }
 
+void TileManager::ResetDamagedRect() { damage_rect_ = gfx::Rect(0, 0, 0, 0); }
+
+gfx::Rect TileManager::GetDamagedRect() { return damage_rect_; }
+
 void TileManager::DidChangeTilePriority(Tile* tile) {
   prioritized_tiles_dirty_ = true;
 }
@@ -1232,6 +1237,9 @@ void TileManager::OnRasterTaskCompleted(
     return;
   }
 
+  // Union damage rect with current tile's content rect.
+  damage_rect_ = gfx::UnionRects(damage_rect_, tile->content_rect());
+
   ++update_visible_tiles_stats_.completed_count;
 
   tile_version.set_has_text(analysis.has_text);