cc: Add const versions of operator* to layer iterators.

cc/layers/picture_layer_impl.h

Index: cc/layers/picture_layer_impl.h
diff --git a/cc/layers/picture_layer_impl.h b/cc/layers/picture_layer_impl.h
index 3b07c5dfd367897ce1e7b7524846c66c1908072a..d40285a12765905e79f05952980a9bc3fb162f20 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -34,6 +34,7 @@ class CC_EXPORT PictureLayerImpl
     ~LayerRasterTileIterator();
 
     Tile* operator*();
+    const Tile* operator*() const;
     LayerRasterTileIterator& operator++();
     operator bool() const;
 
@@ -62,6 +63,7 @@ class CC_EXPORT PictureLayerImpl
     ~LayerEvictionTileIterator();
 
     Tile* operator*();
+    const Tile* operator*() const;
     LayerEvictionTileIterator& operator++();
     operator bool() const;