Backport PNG output format to origin/master branch.

samples/image_diff_png.h

Index: samples/image_diff_png.h
diff --git a/samples/image_diff_png.h b/samples/image_diff_png.h
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+// Copyright 2013 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 TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
+#define TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
+
+#include <stdlib.h>  // for size_t.
+
+#include <vector>
+
+namespace image_diff_png {
+
+// Decode a PNG into an RGBA pixel array.
+bool DecodePNG(const unsigned char* input, size_t input_size,
+               std::vector<unsigned char>* output,
+               int* width, int* height);
+
+// Encode an RGBA pixel array into a PNG.
+bool EncodeRGBAPNG(const unsigned char* input,
+                   int width,
+                   int height,
+                   int row_byte_width,
+                   std::vector<unsigned char>* output);
+
+// Encode an BGRA pixel array into a PNG.
+bool EncodeBGRAPNG(const unsigned char* input,
+                   int width,
+                   int height,
+                   int row_byte_width,
+                   bool discard_transparency,
+                   std::vector<unsigned char>* output);
+
+}  // namespace image_diff_png
+
+#endif  // TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_