Linux: add GYP variable to use the system libpng.

base/gfx/png_decoder.cc

 // Copyright (c) 2006-2008 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.
 
 #include "base/gfx/png_decoder.h"
 
 #include "base/logging.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 
 extern "C" {
+#if defined(USE_SYSTEM_LIBPNG)
+#include <png.h>
+#else
 #include "third_party/libpng/png.h"
+#endif
 }
 
 namespace {
 
 // Converts BGRA->RGBA and RGBA->BGRA.
 void ConvertBetweenBGRAandRGBA(const unsigned char* input, int pixel_width,
                                unsigned char* output) {
   for (int x = 0; x < pixel_width; x++) {
     const unsigned char* pixel_in = &input[x * 4];
     unsigned char* pixel_out = &output[x * 4];
@@ skipping 342 matching lines @@
     }
     bitmap_data[i + 3] = alpha;
     bitmap_data[i] = (bgra[i] * alpha) >> 8;
     bitmap_data[i + 1] = (bgra[i + 1] * alpha) >> 8;
     bitmap_data[i + 2] = (bgra[i + 2] * alpha) >> 8;
   }
 
   bitmap->setIsOpaque(opaque);
   return bitmap;
 }