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

Failing on Mac waterfall, suspecting:
https://chromium-review.googlesource.com/c/chromium/src/+/1749135

This test uses setTimeout(..., 500) which should be fixed.

TBR: tonikitoo@igalia.com
Bug: 993328
Change-Id: I1909991c4e7df4ca75c4b02ef1fc9b917b27b4db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752566Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686686}

third_party/blink/web_tests/TestExpectations

@@ -6460,3 +6460,6 @@ crbug.com/856601 [ Linux ] external/wpt/secure-contexts/idlharness.any.html [ Pa
 #Sheriff 2019-08-12
 crbug.com/783154 [ Linux Debug ] virtual/audio-service/media/controls/doubletap-to-jump-backwards.html [ Failure ]
 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 ]