Update from https://crrev.com/304418

cc/test/pixel_test_utils.h

 // Copyright (c) 2012 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 CC_TEST_PIXEL_TEST_UTILS_H_
 #define CC_TEST_PIXEL_TEST_UTILS_H_
 
+#include <string>
+
 #include "base/files/file_path.h"
 #include "cc/test/pixel_comparator.h"
 
 class SkBitmap;
 
 namespace cc {
 
 // Encodes a bitmap into a PNG and write to disk. Returns true on success. The
 // parent directory does not have to exist.
 bool WritePNGFile(const SkBitmap& bitmap, const base::FilePath& file_path,
     bool discard_transparency);
 
 // Reads and decodes a PNG image to a bitmap. Returns true on success. The PNG
 // should have been encoded using |gfx::PNGCodec::Encode|.
 bool ReadPNGFile(const base::FilePath& file_path, SkBitmap* bitmap);
 
 std::string GetPNGDataUrl(const SkBitmap& bitmap);
 
 // Compares with a PNG file on disk using the given PixelComparator, and returns
 // true if the comparator returns a match. |ref_img_path| is absolute.
 bool MatchesPNGFile(const SkBitmap& gen_bmp,
                     base::FilePath ref_img_path,
                     const PixelComparator& comparator);
 
 }  // namespace cc
 
 #endif  // CC_TEST_PIXEL_TEST_UTILS_H_