file_manager: Rename fileSystemId to volumeId for requestFileSystem()

Along the way, update the comment of this function.

BUG=322339
TEST=none

Review URL: https://codereview.chromium.org/82503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236748 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236748