Mark virtual/audio-service/media/video-zoom.html as flaky

Looks like this test is inherently flaky (setTimeout) and something
made it start flaking more frequently.

TBR=guidou@chromium.org

Bug: 993669
Change-Id: I1757b80c9c54031e2a54d8388c1c39b25190980e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752501Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686722}

third_party/blink/web_tests/TestExpectations

@@ -6454,3 +6454,4 @@ crbug.com/993238 fast/frames/frame-name-reset.html [ Pass Failure ]
 
 # Sheriff 2019-08-14
 crbug.com/993328 [ Mac ] virtual/audio-service/media/video-currentTime.html [ Pass Failure ]
+crbug.com/993669 [ Mac ] virtual/audio-service/media/video-zoom.html [ Pass Failure ]