Rename text track methods to make way for audio & video tracks.

- Rename various text track related methods to explicitly indicate that
  they apply to text tracks. This is to avoid confusion when audio &
  video track logic is landed.

- Renamed TextTrackList::clearOwner() to clearOwnerAndClients() and moved
  some the client clearing out of HTMLMediaElement and into TextTrackList.
  This doesn't introduce any change in behavior. It just restores some
  encapsulation.

BUG=249427
TEST=All LayoutTests still pass.

Review URL: https://codereview.chromium.org/180173002

git-svn-id: svn://svn.chromium.org/blink/trunk@167826 bbb929c8-8fbe-4397-9dbb-9b2b20218538