Downstream expectations will also need fixing when transitioning MseBufferByPts to on-by-default (they're currently verifying ByDts buffered ranges, for example).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/04b4b465d879de468d3c2dbed32130926c2da0cd

commit 04b4b465d879de468d3c2dbed32130926c2da0cd
Author: Matt Wolenetz <wolenetz@chromium.org>
Date: Thu Apr 26 17:21:53 2018

MseBufferByPts: Add field trial testing config

Once MseBufferByPts feature variation experiments begin on Beta/Stable,
this corresponding field trial testing config will need to be in place.

This change adds that config in advance of Beta/Stable experimentation
to identify waterfall regressions ASAP.

Note: Some MSE layout tests will still need updating to match
MseBufferByPts behavior (bug 807793); the field trial testing config is
not used for layout tests (from local experimentation).

BUG=760264,807793
TEST=Local Chromium build has MseBufferByPts enabled by default. Local
MSE layout tests still meet current expectations.
R=mpearson@chromium.org

Change-Id: I9c54b82efe326b3efc84faf51f2b0b928df67c82
Reviewed-on: https://chromium-review.googlesource.com/1028972
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554062}
[modify] https://crrev.com/04b4b465d879de468d3c2dbed32130926c2da0cd/testing/variations/fieldtrial_testing_config.json

Note to self: see also https://chromium-review.googlesource.com/c/chromium/src/+/1068215/5/third_party/WebKit/LayoutTests/http/tests/media/media-source/mediasource-sequencemode-append-buffer.html#b53