[Cast] Track audio queued in encoder; account for it in ShouldDropNextFrame().

This change is a prerequisite for transitioning the "max outstanding
frames" decision logic over to a time-based heuristic.  It adds tracking
of the number of audio samples enqueued in the encoder, from which the
needed backlog stats can be determined.

Also, this introduces an abstract method in FrameSender,
GetNumberOfFramesInEncoder(), since this is computed differently for
audio versus video.

BUG=404813

Review URL: https://codereview.chromium.org/545593002

Cr-Commit-Position: refs/heads/master@{#293987}

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