Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

cc/tiles/picture_layer_tiling.h

 // 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.
 
 #ifndef CC_TILES_PICTURE_LAYER_TILING_H_
 #define CC_TILES_PICTURE_LAYER_TILING_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 194 matching lines @@
     Tile* operator*() const { return current_tile_; }
 
     CoverageIterator& operator++();
     operator bool() const { return tile_j_ <= bottom_; }
 
     int i() const { return tile_i_; }
     int j() const { return tile_j_; }
 
    private:
     const PictureLayerTiling* tiling_;
+    gfx::Size dest_layer_bounds_;
     gfx::Rect dest_rect_;
     float dest_to_content_scale_;
 
     Tile* current_tile_;
     gfx::Rect current_geometry_rect_;
     int tile_i_;
     int tile_j_;
     int left_;
     int top_;
     int right_;
@@ skipping 129 matching lines @@
   bool has_eventually_rect_tiles_;
   bool all_tiles_done_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PictureLayerTiling);
 };
 
 }  // namespace cc
 
 #endif  // CC_TILES_PICTURE_LAYER_TILING_H_