cc: Drop an unused member variable from tiling data iterators

Tiling data iterators need tiling data pointer only in constructors thus
there is no need to store that pointer into a member variable.

Review URL: https://codereview.chromium.org/798973003

Cr-Commit-Position: refs/heads/master@{#308330}

cc/base/tiling_data.h

@@ -76,13 +76,12 @@ class CC_EXPORT TilingData {
     }
 
    protected:
-    explicit BaseIterator(const TilingData* tiling_data);
+    BaseIterator();
     void done() {
       index_x_ = -1;
       index_y_ = -1;
     }
 
-    const TilingData* tiling_data_;
     int index_x_;
     int index_y_;
   };