MSE: Remove downstream mediasource-errors.html web_test

Inspection of upstream external/wpt/media-source/mediasource-errors.html
and downstream http/tests/media/media-source/mediasource-errors.html
shows that upstream is a working, more up to date, superset of the
downstream cases.  Especially with recent landing of
https://chromium-review.googlesource.com/c/chromium/src/+/1371166 which
fixed a timing/flakiness regression in upstream, removal of the unfixed,
less complete, downstream version of that suite is motivated.

BUG=913775, b/35361540
CC=chadduffin@chromium.org
R=sandersd@chromium.org

Change-Id: Ia2f7af9061d17f5a084cd0dfd2a0a98ec92012ce
Reviewed-on: https://chromium-review.googlesource.com/c/1372661
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615750}
1 file changed