Include thread_checker.h in connection_unittests.cc

TBR=rjwright@chromium.org

Review-Url: https://codereview.chromium.org/2653853002 .
Cr-Commit-Position: refs/heads/master@{#445572}

remoting/protocol/connection_unittest.cc

@@ -12,6 +12,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread.h"
+#include "base/threading/thread_checker.h"
 #include "remoting/base/constants.h"
 #include "remoting/proto/audio.pb.h"
 #include "remoting/protocol/audio_source.h"