Allow task pools to reason about transparency.

cc/tiles/tile.h

Index: cc/tiles/tile.h
diff --git a/cc/tiles/tile.h b/cc/tiles/tile.h
index 083c76314a70af63b0c91afffcb392185ed68ca5..e245e0111d9ffa3fb26ed09e560d4e1d006eee41 100644
--- a/cc/tiles/tile.h
+++ b/cc/tiles/tile.h
@@ -43,7 +43,7 @@ class CC_EXPORT Tile {
           contents_scale(contents_scale) {}
   };
 
-  enum TileRasterFlags { USE_PICTURE_ANALYSIS = 1 << 0 };
+  enum TileRasterFlags { USE_PICTURE_ANALYSIS = 1 << 0, IS_OPAQUE = 1 << 1 };
 
   typedef uint64 Id;
 
@@ -65,6 +65,8 @@ class CC_EXPORT Tile {
     return !!(flags_ & USE_PICTURE_ANALYSIS);
   }
 
+  bool is_opaque() const { return !!(flags_ & IS_OPAQUE); }
+
   void AsValueInto(base::trace_event::TracedValue* value) const;
 
   const TileDrawInfo& draw_info() const { return draw_info_; }