Don't allow null bytes in hosts of host permissions.

      BUG=390624
TEST=Load the sample manifest from the bug, comment #9. It should fail to load.

Review URL: https://codereview.chromium.org/416263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285492 0039d316-1c4b-4281-b951-d872f2087c98