Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

cc/base/tiling_data.cc

Index: cc/base/tiling_data.cc
diff --git a/cc/base/tiling_data.cc b/cc/base/tiling_data.cc
index cf2bb57f64c623a09329ee9eecc257358b3bdd7f..480888f92d1d201b96f9381511e7c5f912a51b30 100644
--- a/cc/base/tiling_data.cc
+++ b/cc/base/tiling_data.cc
@@ -7,6 +7,7 @@
 #include <algorithm>
 
 #include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/rect_f.h"
 #include "ui/gfx/geometry/vector2d.h"
 
 namespace cc {
@@ -279,6 +280,12 @@ int TilingData::TileSizeY(int y_index) const {
   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_;