modify test_shell target to build everything for layout_tests, move image_diff to tools/

tools\image_diff\image_diff.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.
 
 // This file input format is based loosely on
 // WebKitTools/DumpRenderTree/ImageDiff.m
 
 // The exact format of this tool's output to stdout is important, to match
 // what the run-webkit-tests script expects.
 
@@ skipping 344 matching lines @@
                         WideToUTF8(values[1]).c_str(),
                         WideToUTF8(values[2]).c_str());
     }
   } else if (values.size() == 2) {
     return CompareImages(argv[1], argv[2]);
   }
 
   PrintHelp();
   return kStatusError;
 }