Mark activation-behavior.window.html layout test as flaky on Mac

BUG=824848
TBR=dom-dev@chromium.org,qyearsley@chromium.org

Change-Id: I9b80a22d0084a151fb13e41ebb9bf2f409160cc4
Reviewed-on: https://chromium-review.googlesource.com/c/1461683Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630613}

third_party/blink/web_tests/TestExpectations

@@ -5112,6 +5112,7 @@ crbug.com/824775 [ Win ] virtual/video-surface-layer/media/controls/video-contro
 crbug.com/824775 [ Mac ] virtual/video-surface-layer/media/controls/video-controls-with-cast-rendering.html [ Pass Failure ]
 crbug.com/824830 fast/peerconnection/RTCPeerConnection-many.html [ Timeout Crash Failure Pass ]
 crbug.com/824848 [ Linux ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
+crbug.com/824848 [ Mac ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
 crbug.com/824848 [ Win ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
 crbug.com/824904 inspector-protocol/heap-profiler/heap-objects-tracking.js [ Pass Failure ]