cc: Add tiling raster tile iterators.

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 4852110e63715f1026ffde77f57ffd58492f2d19..d8cb29b838b477f1499f80ca04826621df6ba7cd 100644
--- a/cc/base/tiling_data.cc
+++ b/cc/base/tiling_data.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 
+#include "cc/base/util.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/vector2d.h"
 
@@ -72,10 +73,12 @@ std::pair<int, int> TilingData::UnclampedFirstBorderTileIndexFromSrcCoord(
     int x,
     int y) const {
   int inner_tile_width = max_texture_size_.width() - 2 * border_texels_;
-  int result_x = (x - 2 * border_texels_) / inner_tile_width;
+  int result_x =
+      RoundDown(x - 2 * border_texels_, inner_tile_width) / inner_tile_width;
 
   int inner_tile_height = max_texture_size_.height() - 2 * border_texels_;
-  int result_y = (y - 2 * border_texels_) / inner_tile_height;
+  int result_y =
+      RoundDown(y - 2 * border_texels_, inner_tile_height) / inner_tile_height;
 
   return std::make_pair(result_x, result_y);
 }
@@ -84,10 +87,10 @@ std::pair<int, int> TilingData::UnclampedLastBorderTileIndexFromSrcCoord(
     int x,
     int y) const {
   int inner_tile_width = max_texture_size_.width() - 2 * border_texels_;
-  int result_x = x / inner_tile_width;
+  int result_x = RoundDown(x, inner_tile_width) / inner_tile_width;
 
   int inner_tile_height = max_texture_size_.height() - 2 * border_texels_;
-  int result_y = y / inner_tile_height;
+  int result_y = RoundDown(y, inner_tile_height) / inner_tile_height;
 
   return std::make_pair(result_x, result_y);
 }
@@ -296,6 +299,8 @@ TilingData::BaseIterator::BaseIterator(const TilingData* tiling_data)
       index_y_(-1) {
 }
 
+TilingData::Iterator::Iterator() : BaseIterator(NULL) { done(); }
+
 TilingData::Iterator::Iterator(const TilingData* tiling_data,
                                const gfx::Rect& tiling_rect)
     : BaseIterator(tiling_data),
@@ -433,6 +438,11 @@ TilingData::DifferenceIterator& TilingData::DifferenceIterator::operator++() {
   return *this;
 }
 
+TilingData::SpiralDifferenceIterator::SpiralDifferenceIterator()
+    : BaseIterator(NULL) {
+  done();
+}
+
 TilingData::SpiralDifferenceIterator::SpiralDifferenceIterator(
     const TilingData* tiling_data,
     const gfx::Rect& consider_rect,