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MseBufferByPts was enabled by default in M75. This series of changes (in M76)
removes:

1) (previous CL) the tests for the old "LegacyByDts" MSE buffering logic, and
2) (this change) the old "LegacyByDts" MSE buffering logic, the
   templating of SourceBufferStream by the buffering implementation, and the
   associated feature gating

Later changes will remove residual complications such as the split
SourceBufferRange and SourceBufferRangeByPts types and clean up
confusing usage of DecodeTimestamp where it really contains a
presentation timestamp within and below FrameProcessor.

BUG=771349,760264,718641,398141

Change-Id: I93491bd47e8cd498b4452f9e5109eb9a7c7b9852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1586461Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658819}