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

tools/image_diff/image_diff.gyp

Index: tools/image_diff/image_diff.gyp
===================================================================
--- tools/image_diff/image_diff.gyp	(revision 0)
+++ tools/image_diff/image_diff.gyp	(revision 0)
@@ -0,0 +1,23 @@
+# Copyright (c) 2009 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.
+
+{
+  'includes': [
+    '../../build/common.gypi',
+  ],
+  'targets': [
+    {
+      'target_name': 'image_diff',
+      'type': 'executable',
+      'msvs_guid': '50B079C7-CD01-42D3-B8C4-9F8D9322E822',
+      'dependencies': [
+        '../../base/base.gyp:base',
+        '../../base/base.gyp:base_gfx',
+      ],
+      'sources': [
+        'image_diff.cc',
+      ],
+    },
+  ]
+}