Support alternate audio/video track ids in ChunkDemuxer tests

Also added a new unit test case where audio and video track ids change
in the second init segment, and did some minor refactoring (renamed
InitSegmentReceived -> InitSegmentReceivedMock).

Review URL: https://codereview.chromium.org/1860263002

Cr-Commit-Position: refs/heads/master@{#386248}
1 file changed