The caller expects GetValidString() to overwrite its outparam, so clear that
outparam on entry so that that's what will actually happen.

BUG=326527
TEST=In debug mode, paste "ab:8/;" into the omnibox and ensure no DCHECK failure happens
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/157403008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250618