Add PluginMessageLoopProxy and use it for Host plugin UI thread.

remoting/remoting.gyp

 # Copyright (c) 2011 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.
 
 {
   'variables': {
     # TODO(dmaclach): can we pick this up some other way? Right now it's
     # duplicated from chrome.gyp
     'chromium_code': 1,
     # Use a consistent MIME-type independent of branding.
@@ skipping 308 matching lines @@
         'base/decompressor.h',
         'base/decompressor_verbatim.cc',
         'base/decompressor_verbatim.h',
         'base/decompressor_zlib.cc',
         'base/decompressor_zlib.h',
         'base/encoder.h',
         'base/encoder_vp8.cc',
         'base/encoder_vp8.h',
         'base/encoder_row_based.cc',
         'base/encoder_row_based.h',
+        'base/plugin_message_loop_proxy.cc',
+        'base/plugin_message_loop_proxy.h',
         'base/rate_counter.cc',
         'base/rate_counter.h',
         'base/running_average.cc',
         'base/running_average.h',
         'base/task_thread_proxy.cc',
         'base/task_thread_proxy.h',
         'base/tracer.cc',
         'base/tracer.h',
         'base/types.h',
         'base/util.cc',
@@ skipping 484 matching lines @@
         ['target_arch=="arm"', {
           'sources!': [
             'base/decoder_vp8_unittest.cc',
             'base/encoder_vp8_unittest.cc',
           ],
         }],
       ],  # end of 'conditions'
     },  # end of target 'remoting_unittests'
   ],  # end of targets
 }