Dirty rects always contain full tiles with delegated rendering.

cc/resources/caching_bitmap_content_layer_updater.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/resources/caching_bitmap_content_layer_updater.h"
 
 #include "base/logging.h"
 #include "cc/resources/layer_painter.h"
 #include "skia/ext/platform_canvas.h"
 
@@ skipping 16 matching lines @@
     int layer_id)
     : BitmapContentLayerUpdater(painter.Pass(),
                                 stats_instrumentation,
                                 layer_id),
       pixels_did_change_(false) {}
 
 CachingBitmapContentLayerUpdater::~CachingBitmapContentLayerUpdater() {}
 
 void CachingBitmapContentLayerUpdater::PrepareToUpdate(
     gfx::Rect content_rect,
+    gfx::Rect paint_rect,
     gfx::Size tile_size,
     float contents_width_scale,
     float contents_height_scale,
     gfx::Rect* resulting_opaque_rect) {
   BitmapContentLayerUpdater::PrepareToUpdate(content_rect,
+                                             paint_rect,
                                              tile_size,
                                              contents_width_scale,
                                              contents_height_scale,
                                              resulting_opaque_rect);
 
   const SkBitmap& new_bitmap = canvas_->getDevice()->accessBitmap(false);
   SkAutoLockPixels lock(new_bitmap);
   DCHECK_GT(new_bitmap.bytesPerPixel(), 0);
   pixels_did_change_ = new_bitmap.config() != cached_bitmap_.config() ||
                        new_bitmap.height() != cached_bitmap_.height() ||
                        new_bitmap.width() != cached_bitmap_.width() ||
                        memcmp(new_bitmap.getPixels(),
                               cached_bitmap_.getPixels(),
                               new_bitmap.getSafeSize());
 
   if (pixels_did_change_)
     new_bitmap.deepCopyTo(&cached_bitmap_, new_bitmap.config());
 }
 
 }  // namespace cc