GetHelp manifest.json update for M34. Also enable it in Guest mode.

BUG=chrome:334769
TEST=manual

Review URL: https://codereview.chromium.org/169343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251698 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251698