Enable proxy_unittest (thread proxy tests).

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1876773002

Cr-Commit-Position: refs/heads/master@{#12314}
diff --git a/webrtc/api/proxy_unittest.cc b/webrtc/api/proxy_unittest.cc
index 85e5438..2578e9b 100644
--- a/webrtc/api/proxy_unittest.cc
+++ b/webrtc/api/proxy_unittest.cc
@@ -12,7 +12,7 @@
 
 #include <string>
 
-#include "testing/base/public/gmock.h"
+#include "testing/gmock/include/gmock/gmock.h"
 #include "webrtc/base/gunit.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/scoped_ptr.h"