Disable http/tests/media/video-throttled-load-metadata.html on Mac

It is flaky on mac_chromium_rel_ng

Bug: 870259
No-try: True
No-presubmit: True
Change-Id: I73ac22cce3d7d03ce2f36c90d79e8a829fe28100
Reviewed-on: https://chromium-review.googlesource.com/1160543Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580191}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4863,3 +4863,6 @@ crbug.com/869364 virtual/gpu/fast/canvas/OffscreenCanvas-filter.html [ Pass Time
 crbug.com/869773 [ Mac Linux ] http/tests/misc/window-dot-stop.html [ Pass Failure ]
 crbug.com/869829 [ Mac Release ] http/tests/media/video-cancel-load.html [ Pass Failure ]
 crbug.com/862806 [ Linux ] css3/filters/effect-drop-shadow-hw.html [ Pass Timeout ]
+
+# Sheriff 2018-08-02
+crbug.com/870259 [ Mac ] http/tests/media/video-throttled-load-metadata.html [ Pass Timeout ]