Make generated bookmark apps have no web extent.

This patch removes web extents from bookmark apps and stops them from
using the launch url as the web extent by default. This was preventing
creation of bookmark apps for any page that was in an overlapping extent
(e.g it wasn't possible to create a bookmark app for drive.google.com/keep
while the default Google Drive app was installed).

This doesn't affect the hosted app security model because bookmark apps
don't use the hosted app process model and have no permissions.

BUG=318607
TBR=dbeam@chromium.org

Review URL: https://codereview.chromium.org/137513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245983 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245983