Bug 1396493 - Part6 - Fix unified build error due to adding a new gtest unit. r=cpearce

MozReview-Commit-ID: 8l3N9c8emDb

--HG--
extra : rebase_source : e85c543e35585a5bb3c291bf8bb9c6c44be3f7d6

dom/media/gtest/TestMediaEventSource.cpp

@@ -5,6 +5,7 @@
 
 #include "gtest/gtest.h"
 
+#include "mozilla/SharedThreadPool.h"
 #include "mozilla/TaskQueue.h"
 #include "mozilla/UniquePtr.h"
 #include "MediaEventSource.h"