fixed uninit error in chromedriver unittests Device::ForwardDevtoolsPort

BUG=393231
R=frankf@chromium.org
TEST=Dr.Memory

Review URL: https://codereview.chromium.org/388863003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282673 0039d316-1c4b-4281-b951-d872f2087c98