Chrome OS: Extract "Downloads" folder mounting code into one place.

This is a prepartion for multi-profile support in Files.app.

Current implementation gives the same filesystem URL for all Downloads folder
from different profiles, and thus cross-profile file operations cannot determine
where each file came from. This CL extracts the code giving the URL (or the code
does mounting, in FileSystem API jargon) for the folder into one place so that
we can consistently apply the coming change.

Actual switch will come later once the Files.app became ready for it.

BUG=336090

Review URL: https://codereview.chromium.org/142983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246168 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246168