Add is_ephemeral flag to the profile cache.

This is needed to be able to clean-up stale ephemeral profiles on start-up after
a crash.
Also ephemeral profiles should never be set as the last profile because they
should not be opened on next start-up since they will be gone by then.

BUG=302461
TEST=none (will follow up when this new flag is used).

Review URL: https://codereview.chromium.org/27202002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229848 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229848