Disable http/tests/media/video-cancel-load.html on mac_chromium_rel_ng

It is flaky

Bug: 869829
No-try: True
No-presubmit: True
Change-Id: I9f49e4c6c7d3457e967e964ee1f8e5e36402a258
Reviewed-on: https://chromium-review.googlesource.com/1158624Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579781}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4867,3 +4867,4 @@ crbug.com/869364 virtual/gpu/fast/canvas/OffscreenCanvas-filter.html [ Pass Time
 # Sheriff 2018-08-01
 crbug.com/869773 [ Mac Linux ] http/tests/misc/window-dot-stop.html [ Pass Failure ]
 crbug.com/869800 [ Mac Release ] virtual/outofblink-cors/http/tests/xmlhttprequest/timeout/xmlhttprequest-timeout-worker-aborted.html [ Pass Timeout ]
+crbug.com/869829 [ Mac Release ] http/tests/media/video-cancel-load.html [ Pass Failure ]