Make AudioDebugFileWriter::AudioFileWriter use a SequencedTaskRunner

Quoting gab@ from

AudioFileWriter::AudioFileWriter() merely takes the task runner to assert it's
running on it, that can easily be turned into a SequencedTaskRunner but even
better than that would be to have it use a SequenceChecker to check that all of
its methods run in sequence (will need to DetachFromSequence in
AudioFileWriter::AudioFileWriter() as it's not constructed on same sequence it's
used -- see sequence_checker.h for details).

SequenceChecker + implicit AssertIOAllowed() calls from using base file methods
guarantees this class is being used in sequence with MayBlock() trait. That's
preferred and cleaner than taking a task_runner just to assume running on it.

Then there's OnThreadDeleter which could easily be made OnSequenceDeleter +


Cr-Commit-Position: refs/heads/master@{#473264}
2 files changed