Revert "Add image_diff from the chromium repo."

sky/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/ui.gni")
 import("//tools/grit/grit_rule.gni")
 
 group("sky") {
   testonly = true
 
   deps = [
     "//sky/engine/platform:platform_unittests",
     "//sky/engine/web:sky_unittests",
     "//sky/engine/wtf:unittests",
     "//sky/services/inspector",
     "//sky/tools/debugger",
-    "//sky/tools/imagediff",
     "//sky/tools/tester",
     "//sky/viewer",
   ]
   if (!is_android) {
     deps += [ "//third_party/mesa:osmesa" ]
   }
 }