Represent kiosk apps as user pods instead of menu items.

Add a custom user pod icon for such apps.
Enable this UI by default.
Provide --disable-new-kiosk-ui flag which reverts back to the original UI (menu).

BUG=334304,341884
R=xiyuan@chromium.org
TBR=oshima

Review URL: https://codereview.chromium.org/158833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251315 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251315