app_shell: Enable process-per-site

This ensures that all render frames created for a single app will
use the same render process.

BUG=346410
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/180243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253305 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253305