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}
diff --git a/third_party/blink/web_tests/TestExpectations b/third_party/blink/web_tests/TestExpectations
index 7b61b51d9..dad6215 100644
--- a/third_party/blink/web_tests/TestExpectations
+++ b/third_party/blink/web_tests/TestExpectations
@@ -5717,4 +5717,4 @@
 
 # Sheriff 2019-06-05
 crbug.com/971031 [ Mac ] fast/dom/timer-throttling-hidden-page.html [ Pass Failure ]
-
+crbug.com/971252 [ Mac ] external/wpt/media-source/mediasource-play.html [ Pass Timeout ]