Mark external/wpt/media-source/mediasource-play.html timeout on Mac

TBR=gayane@chromium.org
NOTRY=true

Bug: 971252
Change-Id: I1b93944ca76b422c7b8c4972e161326717a612c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645475
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666278}
1 file changed