Wire the ephemeral mode policy to a pref and register the pref from inside ProfileImpl.

BUG=298944
TEST=none

Review URL: https://codereview.chromium.org/25258002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226184 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226184