app_shell: Allow chrome-extension scheme to request filesystems

BUG=347573
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/183323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253846 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253846