Add switch 'enable-ephemeral-apps'

Added switch to hide experimental ephemeral apps. This will appear in
chrome://flags

BUG=None

Review URL: https://codereview.chromium.org/27516006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229771 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229771