Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

cc/base/tiling_data.cc

 // Copyright 2010 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/base/tiling_data.h"
 
 #include <algorithm>
 
 #include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/rect_f.h"
 #include "ui/gfx/geometry/vector2d.h"
 
 namespace cc {
 
 static int ComputeNumTiles(int max_texture_size,
                            int total_size,
                            int border_texels) {
   if (max_texture_size - 2 * border_texels <= 0)
     return total_size > 0 && max_texture_size >= total_size ? 1 : 0;
 
@@ skipping 252 matching lines @@
     return max_texture_size_.height() - border_texels_;
   if (y_index < num_tiles_y_ - 1)
     return max_texture_size_.height() - 2 * border_texels_;
   if (y_index == num_tiles_y_ - 1)
     return tiling_size_.height() - TilePositionY(y_index);
 
   NOTREACHED();
   return 0;
 }
 
+gfx::RectF TilingData::TexelExtent(int i, int j) const {
+  gfx::RectF result(TileBoundsWithBorder(i, j));
+  result.Inset(0.5f, 0.5f);
+  return result;
+}
+
 gfx::Vector2d TilingData::TextureOffset(int x_index, int y_index) const {
   int left = (!x_index || num_tiles_x_ == 1) ? 0 : border_texels_;
   int top = (!y_index || num_tiles_y_ == 1) ? 0 : border_texels_;
 
   return gfx::Vector2d(left, top);
 }
 
 void TilingData::RecomputeNumTiles() {
   num_tiles_x_ = ComputeNumTiles(
       max_texture_size_.width(), tiling_size_.width(), border_texels_);
@@ skipping 566 matching lines @@
 
     // We should always end up in an around rect at some point.
     // Since the direction is now vertical, we have to ensure that we will
     // advance.
     DCHECK_GE(horizontal_step_count_, 1);
     DCHECK_GE(vertical_step_count_, 1);
   }
 }
 
 }  // namespace cc