[Mac] Return an empty StringPiece from GetDataResource() when g_resource_data_pack is NULL.

webkit/support/platform_support_mac.mm

 // Copyright (c) 2010 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 "webkit/support/platform_support.h"
 
 #import <AppKit/AppKit.h>
 #import <Foundation/Foundation.h>
 #import <objc/objc-runtime.h>
 
@@ skipping 193 matching lines @@
       path = path.AppendASCII("textAreaResizeCorner.png");
       bool success = file_util::ReadFileToString(path, &resize_corner_data);
       if (!success) {
         LOG(FATAL) << "Failed reading: " << path.value();
       }
     }
     return resize_corner_data;
   }
   }
   base::StringPiece res;
-  g_resource_data_pack->GetStringPiece(resource_id, &res);
+  if (g_resource_data_pack)
+    g_resource_data_pack->GetStringPiece(resource_id, &res);
   return res;
 }
 
 }  // namespace webkit_glue