Use volumeId instead of mountPath in fileBrrowserPrivate.getSizeStats().

This patch removes the last dependency on deprecated mountPath from private methods.

TEST=Tested manually on Drive and Downloads.
BUG=316077

Review URL: https://codereview.chromium.org/149443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248156 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248156