file_manager: Rename formatDevice to formatVolume

As this function is used to format a volume (a partition on a storage device),
rather than a device.

BUG=316526
TEST=none
R=benwells@chromium.org

Review URL: https://codereview.chromium.org/65503004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236095 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236095