Simplify the Ephemeral Profile test for the ProfileManager browser test suite.

It was timing out regularly on the Windows bots.

BUG=314905
TEST=ProfileManagerBrowserTest.EphemeralProfile doesn't flake so often.

Review URL: https://codereview.chromium.org/61743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233613 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233613