MSE: Refactor SourceBufferRangeByDts out of SourceBufferRange

This CL prepares for having another kind of SourceBufferRange, one that
manages continuous buffered intervals by presentation interval.

BUG=718641

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
Change-Id: I54cac7552ff76b86826d29193b9cea4ad5afae62
Reviewed-on: https://chromium-review.googlesource.com/672007
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503054}
8 files changed