- removes potential for ambiguity between liveDelay variable in
  startPlaybackCatchUp function and PlaybackController module when
  reasoning about the code
- new name intended to convey that the player's liveDelay value is used
  throughout this function

This fixes a bug observed with low-latency streams containing one or
more text representations. The bug prevented the liveCatchUpPlaybackRate
from being correctly applied following a buffer stall, leading to
unconstrained drift from the user-specified target latency.

The bug manifested as follows:

1. a buffer stall event occurs, the 'playbackStalled' field on
   StreamController is set to 'true'
2. 'StreamController.startPlaybackCatchUp' is called, it uses
   'StreamController.getBufferLevel' to obtain the lowest current audio,
   video text or fragmented text buffer level
3. in streams with a text or fragmented text representation, the recent
   change to disable text by default resulted in an empty buffer being
   maintained for that representation _unless_ text had been explicitly
   enabled for the player
4. the empty buffer prevents 'StreamController.startPlaybackCatchUp'
   from resetting the 'playbackStalled' field of 'StreamController' to
   'false', further, it assumes a buffer stall is ongoing, and resets
   the 'newRate' to '1.0', preventing latency from being reduced

This change then checks whether text has been enabled and only adds the
stream processors for textual representations to those returned by the
'Stream.getProcessors' if it is.

Tested on Firefox 78.02 & Chrome 84.0.414.79 on Ubuntu 20.04.

- existing test verifies that empty array of stream processors exists
when stream is not activated
- additional tests check that text and fragmented text processors are
returned by Stream.getProcessors _only_ when text is enabled via the
TextController

This fix ensure the correct minimum buffer level is reported when text
is disabled. Previously a buffer level of 0 is reported at all times if
a text track is present in the manifest but text is disabled in the
player. This fix checks whether text is enabled and only includes text
buffer metrics in buffer level reporting if it is.