Fix trychange unittest.

BUG=none
TEST=trychange unittest passes
Review URL: http://codereview.chromium.org/243090

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@27969 0039d316-1c4b-4281-b951-d872f2087c98

tests/trychange_unittest.py

@@ -28,7 +28,7 @@ class TryChangeUnittest(TryChangeTestsBase):
       'RunCommand', 'SCM', 'SVN', 'TryChange', 'USAGE',
       'datetime', 'gcl', 'gclient', 'gclient_scm', 'getpass', 'logging',
       'optparse', 'os', 'shutil', 'socket', 'sys', 'tempfile', 'traceback',
-      'upload', 'urllib',
+      'upload', 'urllib', 'subprocess',
     ]
     # If this test fails, you should add the relevant test.
     self.compareMembers(trychange, members)