Files.app: Remove RootDirectory constants from DirectoryContents.

The constant is used for recording metric.
This CL moves the logic from DirectoryContents to DirectoryModel, where we can use the volume manager to check the volume type of the path.

BUG=302090
TEST=manually

Review URL: https://codereview.chromium.org/85083002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237526 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237526