Make LoadableTextTrack::clearClient not virtual

When TextTrack::clearClient and LoadableTextTrack::clearClient were
added in r99984, both had the virtual keyword and nothing seemed odd.

In r135410 the virtual keyword was dropped from TextTrack::clearClient
because "it is unnecessary", but LoadableTextTrack::clearClient was left
virtual even though it now shadowed (instead of overriding) TextTrack.

The HTMLTrackElement destructor calls LoadableTextTrack::clearClient, so
there's no risk of LoadableTextTrack::m_trackElement becoming stale.

It would be nice to sort this out even more, perhaps by refactoring
LoadableTextTrack into a TextTrackCueProvider or something that doesn't
inherit from TextTrack, but until then add a comment to calm the reader.

BUG=313287

Review URL: https://codereview.chromium.org/54963004

git-svn-id: svn://svn.chromium.org/blink/trunk@161197 bbb929c8-8fbe-4397-9dbb-9b2b20218538