[Sheriff] Mark 3 layout tests as flaky on Mac10.11 and WebKit Mac10.13(retina)

WebKit Mac10.13 (retina)
  virtual/audio-service/media/video-controls-rendering.html

Mac10.11
  media/controls/video-overlay-cast-dark-rendering.html
  virtual/audio-service/media/controls/video-overlay-cast-dark-rendering.html

Bug: 1021407
Change-Id: I73769f1ca62f570ecc7ef8fd629ab1ef5977950d
Tbr: andypaicu@google.com
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898903Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#712468}

third_party/blink/web_tests/TestExpectations

@@ -5761,3 +5761,8 @@ crbug.com/963183 http/tests/devtools/network/network-cookies-pane.js [ Pass Time
 crbug.com/963183 http/tests/devtools/network/preview-searchable.js [ Pass Failure ]
 crbug.com/963183 http/tests/devtools/network/waterfall-header-height-updates.js [ Pass Failure ]
 crbug.com/963183 http/tests/devtools/network/network-search.js [ Pass Timeout Failure ]
+
+# Sheriff 2019-11-04
+crbug.com/1021407 [ Retina Release ] virtual/audio-service/media/video-controls-rendering.html [ Pass Failure ]
+crbug.com/1021407 [ Mac10.11 ] virtual/audio-service/media/controls/video-overlay-cast-dark-rendering.html [ Pass Failure ]
+crbug.com/1021407 [ Mac10.11 ] media/controls/video-overlay-cast-dark-rendering.html [ Pass Failure ]