Files.app: Remove RootDirectory constants from DriveBanners.

Instead of refering RootDirectory constants, this CL uses VolumeInfo and Entry
now. To communicate with C++ side, it uses Entry URL, which is associated with
Entry one-by-one.

BUG=302090
TEST=manually
R=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/91103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237879 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237879