Currently various StreamParsers return output buffers via 3 separate
collections: a queue for audio, a queue for video and a map of queues
keyed by the track id for text tracks. Now that all stream parsers
properly assign track id to each buffer we can unify all those 3
collections into a single BufferQueueMap keyed by track ids.

BUG=341581

Review-Url: https://codereview.chromium.org/2254093002
Cr-Commit-Position: refs/heads/master@{#414274}