Add bug to TODO for handling SkImageShader subrects

cc/playback/discardable_image_map.cc

 // Copyright 2015 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/playback/discardable_image_map.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <limits>
@@ skipping 198 matching lines @@
   void AddPaintImage(const SkRect& rect, const SkPaint& paint) {
     SkShader* shader = paint.getShader();
     if (shader) {
       SkMatrix matrix;
       SkShader::TileMode xy[2];
       SkImage* image = shader->isAImage(&matrix, xy);
       if (image) {
         const SkMatrix& ctm = getTotalMatrix();
         matrix.postConcat(ctm);
         // TODO(ericrk): Handle cases where we only need a sub-rect from the
-        // image.
+        // image. crbug.com/671821
         AddImage(sk_ref_sp(image), SkRect::MakeFromIRect(image->bounds()),
                  MapRect(ctm, rect), matrix, &paint);
       }
     }
   }
 
   std::vector<std::pair<DrawImage, gfx::Rect>>* image_set_;
   const SkRect canvas_bounds_;
   const gfx::Size canvas_size_;
   std::vector<SkPaint> saved_paints_;
@@ skipping 33 matching lines @@
     DiscardableImageMap* image_map,
     const gfx::Size& bounds)
     : image_map_(image_map),
       metadata_canvas_(image_map->BeginGeneratingMetadata(bounds)) {}
 
 DiscardableImageMap::ScopedMetadataGenerator::~ScopedMetadataGenerator() {
   image_map_->EndGeneratingMetadata();
 }
 
 }  // namespace cc