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- Move ExpectRendererInitialization() into StartPipeline* to avoid
  having to call this in most tests.
- Simplify audio/video DemuxerStream creation.

Bug: 1018372
Test: Add new tests
Change-Id: I834b40fe8b475ce94661162edec9ed576d161d25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906256
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713995}