Add FakeClipboard implementation for unit tests.

ui/base/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")
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
@@ skipping 637 matching lines @@
   if (is_android || is_ios) {
     sources -= [
       "device_form_factor_desktop.cc",
     ]
   }
 }
 
 source_set("test_support") {
   testonly = true
   sources = [
+    "test/fake_clipboard.cc",
+    "test/fake_clipboard.h",
     "test/ui_controls.h",
     "test/ui_controls_aura.cc",
     "test/ui_controls_internal_win.cc",
     "test/ui_controls_internal_win.h",
     "test/ui_controls_mac.mm",
     "test/ui_controls_win.cc",
   ]
 
   public_deps = [
     ":base",
@@ skipping 240 matching lines @@
       "//ui/chromeos/resources",
       "//ui/chromeos/strings",
       "//ui/chromeos:ui_chromeos",
       "//ui/events:gesture_detection",
       "//ui/message_center:message_center",
     ]
   }
 }
 }
 # TODO(GYP) Mac (ui_unittest_bundle) and Android (ui_unittests_apk).