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/api_tests.gyp b/webrtc/api/api_tests.gyp
index 7cefd13..e66fa7f 100644
--- a/webrtc/api/api_tests.gyp
+++ b/webrtc/api/api_tests.gyp
@@ -43,7 +43,7 @@
         'peerconnectionendtoend_unittest.cc',
         'peerconnectionfactory_unittest.cc',
         'peerconnectioninterface_unittest.cc',
-        # 'peerconnectionproxy_unittest.cc',
+	'proxy_unittest.cc',
         'rtpsenderreceiver_unittest.cc',
         'statscollector_unittest.cc',
         'test/fakeaudiocapturemodule.cc',