Fix build of pdfium_diff

samples/image_diff_png.h

 // 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_