Rename png_ functions conflicting with chrome.

third_party/libpng16/pngprefix.h

Index: third_party/libpng16/pngprefix.h
diff --git a/third_party/libpng16/pngprefix.h b/third_party/libpng16/pngprefix.h
new file mode 100644
index 0000000000000000000000000000000000000000..8eab668297d4a3e1c1a932211d4d3ec7872f3fb1
--- /dev/null
+++ b/third_party/libpng16/pngprefix.h
@@ -0,0 +1,17 @@
+// Copyright 2016 PDFium 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 PNGPREFIX_H
+#define PNGPREFIX_H
+
+// The purpose of this file is to rename conflicting functions
+// when this version of libpng and chromium's version of it are
+// both simultaneously present.
+
+#define png_get_uint_32 PDFIUM_png_get_uint_32
+#define png_get_uint_16 PDFIUM_png_get_uint_16
+#define png_get_int_32 PDFIUM_png_get_int_32
+#define png_get_int_16 PDFIUM_png_get_int_16
+
+#endif  // PNGPREFIX_H