[OriginChip] Tweaks to clear the omnibox when navigating back to the NTP.

BUG=329549

Review URL: https://codereview.chromium.org/121343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248635 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248635