MSE: Don't vary kMseBufferByPts in unit tests unnecessarily

Since the kMseBufferByPts feature enabled/disabled state is cached by
ChunkDemuxer's ctor (to coherently configure the MSE portion of all of
the buffering for that ChunkDemuxer and protect against feature status
possibly changing during the lifetime of the ChunkDemuxer), various unit
tests that test layers below (not including ChunkDemuxer) no longer need
to vary the feature state. They already vary how they configure the
system under test directly (like how ChunkDemuxer would).

This change removes unnecessary kMseBufferByPts toggling in these lower
layer tests:
  SourceBufferStateTests
  FrameProcessorTests
  SourceBufferStreamTests

Note: PipelineIntegrationTests and ChunkDemuxerTests must still do
feature toggling to retain coverage.

BUG=718641

Change-Id: I69e8e59d2b198e38e4f4a7f51ce37b70c862745e
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Reviewed-on: https://chromium-review.googlesource.com/676536
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503593}
3 files changed