Fix 'gn check' errors in //ui/* targets.

ui/snapshot/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("//testing/test.gni")
 
 component("snapshot") {
   sources = [
     "screenshot_grabber.cc",
@@ skipping 49 matching lines @@
   ]
 
   deps = [
     ":snapshot",
     "//base",
     "//base/allocator",
     "//base/test:test_support",
     "//skia",
     "//testing/gtest",
     "//ui/base",
+    "//ui/compositor:test_support",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
   ]
 
   if (use_aura) {
     deps += [
       "//ui/aura:test_support",
       "//ui/compositor",
       "//ui/compositor:test_support",
       "//ui/wm",
     ]
   } else {
     sources -= [ "snapshot_aura_unittest.cc" ]
   }
 }