Remove many unused files from //skia/ext

skia/ext/pixel_ref_utils.h

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef SKIA_EXT_PIXEL_REF_UTILS_H_
-#define SKIA_EXT_PIXEL_REF_UTILS_H_
-
-#include <vector>
-
-#include "SkPicture.h"
-#include "SkRect.h"
-
-namespace skia {
-
-class SK_API PixelRefUtils {
- public:
-
-  struct PositionPixelRef {
-    SkPixelRef* pixel_ref;
-    SkRect pixel_ref_rect;
-  };
-
-  static void GatherDiscardablePixelRefs(
-      SkPicture* picture,
-      std::vector<PositionPixelRef>* pixel_refs);
-};
-
-typedef std::vector<PixelRefUtils::PositionPixelRef> DiscardablePixelRefList;
-
-}  // namespace skia
-
-#endif