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_sources.gypi b/ppapi/ppapi_sources.gypi
index 50e84c3f..8c0c531 100644
--- a/ppapi/ppapi_sources.gypi
+++ b/ppapi/ppapi_sources.gypi
@@ -446,6 +446,8 @@
       'tests/test_media_stream_video_track.h',
       'tests/test_memory.cc',
       'tests/test_memory.h',
+      'tests/test_message_handler.cc',
+      'tests/test_message_handler.h',
       'tests/test_message_loop.cc',
       'tests/test_message_loop.h',
       'tests/test_mouse_cursor.cc',