MSE: Adds feature flag for work to fix PTS/DTS compliance

The new feature flag is media::kMseBufferByPts, and is disabled by default
while working on the landing and experimenting with new buffering logic.

BUG=718641

Change-Id: I858287f206fcb702e75cf6d3d10bbfe1503246a9
Reviewed-on: https://chromium-review.googlesource.com/656564
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500649}
2 files changed