Files.app: Add a command-line flag to switch multi-profile support of Files.app.

This CL adds a command-line flag "file-manager-enable-multi-profile".

BUG=335467
TEST=manually

Review URL: https://codereview.chromium.org/140913008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245513 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245513