Remove few more errors in mac scons build.

webkit/port/platform/image-decoders/png/PNGImageDecoder.cpp

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * Portions are Copyright (C) 2001 mozilla.org
  *
  * Other contributors:
  *   Stuart Parmenter <stuart@mozilla.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ skipping 30 matching lines @@
 #include "assert.h"
 
 namespace WebCore {
 
 // Gamma constants.
 const double cMaxGamma = 21474.83;
 const double cDefaultGamma = 2.2;
 const double cInverseGamma = 0.45455;
 
 // Protect against large PNGs. See Mozilla's bug #251381 for more info.
-const long cMaxPNGSize = 1000000L;
+const unsigned long cMaxPNGSize = 1000000UL;
 
 // Called if the decoding of the image fails.
 static void PNGAPI decodingFailed(png_structp png_ptr, png_const_charp error_msg);
 
 // Callbacks given to the read struct.  The first is for warnings (we want to treat a particular warning
 // as an error, which is why we have to register this callback.
 static void PNGAPI decodingWarning(png_structp png_ptr, png_const_charp warning_msg);
 
 // Called when we have obtained the header information (including the size).
 static void PNGAPI headerAvailable(png_structp png_ptr, png_infop info_ptr);
@@ skipping 340 matching lines @@
 {
     if (m_frameBufferCache.isEmpty())
         return;
 
     // Hand back an appropriately sized buffer, even if the image ended up being empty.
     RGBA32Buffer& buffer = m_frameBufferCache[0];
     buffer.setStatus(RGBA32Buffer::FrameComplete);
 }
 
 }