Add tools/imagediff to GN build.

BUILD.gn

 # Copyright (c) 2013 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 is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 77 matching lines @@
     "//third_party/re2",
     "//third_party/smhasher:cityhash",
     "//third_party/smhasher:murmurhash3",
     "//third_party/smhasher:pmurhash",
     "//third_party/snappy",
     "//third_party/speex",
     "//third_party/widevine/cdm:version_h",
     "//third_party/zlib",
     "//third_party:jpeg",
     "//tools/gn",
+    "//tools/imagediff($host_toolchain)",
     "//ui/app_list",
     "//ui/aura",
     "//ui/accessibility",
     "//ui/base",
     "//ui/display",
     "//ui/events",
     "//ui/gfx",
     "//ui/gl",
     "//ui/keyboard",
     "//ui/native_theme",
@@ skipping 102 matching lines @@
     ]
   }
 
   if (use_ozone) {
     deps += [
       "//ui/events/ozone:events_ozone",
       "//ui/events/ozone:events_ozone_evdev",
     ]
   }
 }