Fix uninit read in TestRunner.

Only happens if the testrunner methods are invoked with invalid
arguments. Protect against that.

BUG=344762
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/171423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251994 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251994