Test set up for multi-profiles in FileManagerBrowserTest.

No actual test added yet. This is just an infrastructural preparation.

BUG=341224

Review URL: https://codereview.chromium.org/156183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250104 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250104