Remove unused paintScrollbarThumb impl.

content/content_child.gypi

 # Copyright 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.
 
 {
   'dependencies': [
     '../base/base.gyp:base',
     '../components/tracing.gyp:tracing',
     '../mojo/mojo_base.gyp:mojo_environment_chromium',
     '../mojo/public/mojo_public.gyp:mojo_application_bindings',
@@ skipping 222 matching lines @@
       'child/webmessageportchannel_impl.cc',
       'child/webmessageportchannel_impl.h',
       'child/websocket_bridge.cc',
       'child/websocket_bridge.h',
       'child/websocket_dispatcher.cc',
       'child/websocket_dispatcher.h',
       'child/webthemeengine_impl_android.cc',
       'child/webthemeengine_impl_android.h',
       'child/webthemeengine_impl_default.cc',
       'child/webthemeengine_impl_default.h',
-      'child/webthemeengine_impl_mac.cc',
       'child/webthemeengine_impl_mac.h',
       'child/webthread_impl.cc',
       'child/webthread_impl.h',
       'child/weburlresponse_extradata_impl.cc',
       'child/weburlresponse_extradata_impl.h',
       'child/worker_task_runner.cc',
       'child/worker_task_runner.h',
       'child/worker_thread_task_runner.cc',
       'child/worker_thread_task_runner.h',
     ],
@@ skipping 109 matching lines @@
         }, {
           'dependencies': [
             '../third_party/nss/nss.gyp:nspr',
             '../third_party/nss/nss.gyp:nss',
           ],
         }],
       ],
     }],
   ],
 }