PPAPI: Implement synchronous postMessage

BUG=367896

Review URL: https://codereview.chromium.org/264303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278102 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/ppapi/ppapi_proxy.gypi b/ppapi/ppapi_proxy.gypi
index 117ede6..c775386e 100644
--- a/ppapi/ppapi_proxy.gypi
+++ b/ppapi/ppapi_proxy.gypi
@@ -94,6 +94,8 @@
           'proxy/media_stream_track_resource_base.h',
           'proxy/media_stream_video_track_resource.cc',
           'proxy/media_stream_video_track_resource.h',
+          'proxy/message_handler.cc',
+          'proxy/message_handler.h',
           'proxy/net_address_resource.cc',
           'proxy/net_address_resource.h',
           'proxy/network_list_resource.cc',