[GCM] Build mcs_probe with gcm_unit_tests

This allows trybots to verify that mcs_probe compiles, without actually attempting to run it.

BUG=364257

Review URL: https://codereview.chromium.org/294963005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272066 0039d316-1c4b-4281-b951-d872f2087c98

google_apis/gcm/gcm.gyp

@@ -124,6 +124,7 @@
         '../../net/net.gyp:net_test_support',
         '../../testing/gtest.gyp:gtest',
         '../../third_party/protobuf/protobuf.gyp:protobuf_lite',
+        'mcs_probe',
         'gcm'
       ],
       'sources': [