MSE: Strictly verify expected MediaLog events in ChunkDemuxerTests
MSE: Strictly verify expected MediaLog events in ChunkDemuxerTests

This CL continues to increase strict media unit test coverage of
expected media-internals logs emitted during tests.  In particular, this
CL adds strict MediaLog event verification to ChunkDemuxerTests.

It includes a few new test cases, and refactors many of the previous
single-stream cluster appends to muxed A/V SourceBuffers to use
more well-formed muxed cluster appends.

R=chcunningham@chromium.org,xhwang@chromium.org
TEST=ChunkDemuxerTests
BUG=524597

Review URL: https://codereview.chromium.org/1581003002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369666}

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