fix(text): Fix webvtt offset in sequence mode

[theodab]

When running in sequence mode, we ignore the normal timestamps
of video and audio segments. This lead to problems in some Apple-
encoded webvtt content, which used the X-TIMESTAMP-MAP tag to account
for the timestamp offsets in their video. Thus, those subtitles would
end up 10 seconds offset.
This changes the webvtt parser to ignore the X-TIMESTAMP-MAP when in
sequence mode.

Issue #2337

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

Checklist:

• I have signed the Google CLA https://cla.developers.google.com
• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have verified my change on multiple browsers on different platforms
• I have run ./build/all.py and the build passes
• I have run ./build/test.py and all tests pass

[joeyparrish]

Looks good otherwise. I'm confident in this for VOD, at least, and I suspect we have work to do generally around live HLS that may not be text-specific.