Make markCaptionAndSubtitleTracksAsUnconfigured() do what the box say

Sink the call to configureTextTracks() out of
markCaptionAndSubtitleTracksAsUnconfigured() - this means the latter does
what it advertises and nothing more.
This also allows tightening the scope for when m_processingPreferenceChange
is set to true.

Review URL: https://codereview.chromium.org/541593004

git-svn-id: svn://svn.chromium.org/blink/trunk@181416 bbb929c8-8fbe-4397-9dbb-9b2b20218538