Enable webkit.gyp compiler warnings on mac only.

webkit/api/src/WebImageSkia.cpp

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 39 matching lines @@
     ImageSource source;
     source.setData(PassRefPtr<SharedBuffer>(data).get(), true);
     if (!source.isSizeAvailable())
         return WebImage();
 
     // Frames are arranged by decreasing size, then decreasing bit depth.
     // Pick the frame closest to |desiredSize|'s area without being smaller,
     // which has the highest bit depth.
     const size_t frameCount = source.frameCount();
     size_t index = 0;  // Default to first frame if none are large enough.
-    int frameAreaAtIndex;
+    int frameAreaAtIndex = 0;
     for (size_t i = 0; i < frameCount; ++i) {
         const IntSize frameSize = source.frameSizeAtIndex(i);
         if (WebSize(frameSize) == desiredSize) {
             index = i;
             break;  // Perfect match.
         }
 
         const int frameArea = frameSize.width() * frameSize.height();
         if (frameArea < (desiredSize.width * desiredSize.height))
             break;  // No more frames that are large enough.
@@ skipping 40 matching lines @@
 {
     NativeImagePtr p;
     if (image.get() && (p = image->nativeImageForCurrentFrame()))
         assign(*p);
     else
         reset();
     return *this;
 }
 
 } // namespace WebKit