Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

cc/base/tiling_data.h

Index: cc/base/tiling_data.h
diff --git a/cc/base/tiling_data.h b/cc/base/tiling_data.h
index 3c3937db81541378eb71cdae42ca419e07af0bf0..247b603d0c7c55a05277f826207d244a521b48db 100644
--- a/cc/base/tiling_data.h
+++ b/cc/base/tiling_data.h
@@ -13,6 +13,7 @@
 #include "ui/gfx/geometry/size.h"
 
 namespace gfx {
+class RectF;
 class Vector2d;
 }
 
@@ -61,6 +62,8 @@ class CC_EXPORT TilingData {
   int TileSizeX(int x_index) const;
   int TileSizeY(int y_index) const;
 
+  gfx::RectF TexelExtent(int i, int j) const;
+
   // Difference between TileBound's and TileBoundWithBorder's origin().
   gfx::Vector2d TextureOffset(int x_index, int y_index) const;