Linux: add GYP variable to use the system libpng.

base/gfx/png_encoder.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/basictypes.h"
 #include "base/gfx/png_encoder.h"
 #include "base/logging.h"
 #include "base/scoped_ptr.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/skia/include/core/SkUnPreMultiply.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 203 matching lines @@
         divided[i + 3] = alpha;
       }
       i += bbp;
     }
   }
 
   return Encode(divided.get(),
                 PNGEncoder::FORMAT_RGBA, input.width(), input.height(),
                 input.width() * bbp, discard_transparency, output);
 }