Mark flaky tests on Mac10.13

There are occasional flakes in:
external/wpt/html/semantics/embedded-content/media-elements/ready-states/autoplay-hidden.optional.html

BUG=1004760

Change-Id: I8e5b5598707e68521fbfeda26d3e030a33153055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808865
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697255}

third_party/blink/web_tests/TestExpectations

@@ -3319,7 +3319,7 @@ crbug.com/626703 [ Mac ] external/wpt/css/css-fonts/font-family-name-025.html [
 crbug.com/626703 [ Mac10.11 ] external/wpt/shape-detection/detection-HTMLVideoElement-invalid-state.html [ Timeout ]
 crbug.com/626703 [ Mac10.12 ] external/wpt/webaudio/the-audio-api/the-mediaelementaudiosourcenode-interface/mediaElementAudioSourceToScriptProcessorTest.html [ Timeout ]
 crbug.com/626703 [ Mac10.12 ] external/wpt/html/semantics/embedded-content/media-elements/track/track-element/track-mode.html [ Timeout ]
-crbug.com/626703 [ Mac10.12 ] external/wpt/html/semantics/embedded-content/media-elements/ready-states/autoplay-hidden.optional.html [ Timeout ]
+crbug.com/1004760 [ Mac ] external/wpt/html/semantics/embedded-content/media-elements/ready-states/autoplay-hidden.optional.html [ Timeout ]
 crbug.com/626703 [ Mac10.12 ] external/wpt/html/semantics/embedded-content/media-elements/track/track-element/track-cue-negative-timestamp-events.html [ Timeout ]
 crbug.com/626703 external/wpt/webvtt/rendering/cues-with-video/processing-model/bidi/start_alignment.html [ Failure ]
 crbug.com/626703 [ Retina ] external/wpt/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-top-left.html [ Timeout ]