Mark http/tests/media/media-source/mediasource-duration.html as flaky on Mac

Bug: 910905
Change-Id: If339d5e7a354779a094bb10d693f047cb73e5cba
TBR: dalecurtis@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/1356988Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613048}

third_party/blink/web_tests/TestExpectations

@@ -4396,7 +4396,7 @@ crbug.com/678499 virtual/outofblink-cors-ns/http/tests/security/contentSecurityP
 crbug.com/747751 [ Win ] http/tests/devtools/application-panel/resources-panel-resource-preview.js [ Failure Pass ]
 
 crbug.com/689781 external/wpt/media-source/mediasource-duration.html [ Failure Pass ]
-crbug.com/689781 [ Win ] http/tests/media/media-source/mediasource-duration.html [ Failure Pass ]
+crbug.com/689781 [ Win Mac ] http/tests/media/media-source/mediasource-duration.html [ Failure Pass ]
 
 crbug.com/681468 fast/forms/suggestion-picker/date-suggestion-picker-appearance-zoom125.html [ Failure Pass ]
 crbug.com/681468 fast/forms/suggestion-picker/date-suggestion-picker-appearance-zoom200.html [ Failure Pass ]