MSE: Remove unused SourceBufferRangeBy*::SeekAhead* methods

These 3 methods in each of SBRByPts and SBRByDts are not used by
anything, and are deleted by this change. I found these during
refactoring SBRByPts for PTS/DTS compliance, and split their deletion
into this CL to make compliance CL(s) smaller.

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: I2a50a6e618e6d3bcc2a799bec398ebe752dba690
Reviewed-on: https://chromium-review.googlesource.com/683154
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504210}
4 files changed