Mark compositing/contents-opaque/control-layer.html flaky on Mac10.10

BUG=1026046
TBR=wangxianzhu@chromium.org

Change-Id: Ib36bd8f64b65f2a2d8a51e61ae5ac3e795c73118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1922703Reviewed-by: Matthew Wolenetz <wolenetz@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716401}

third_party/blink/web_tests/TestExpectations

@@ -5857,3 +5857,4 @@ crbug.com/1025437 virtual/not-site-per-process/http/tests/devtools/debugger/fetc
 
 # Sheriff 2019-11-18
 crbug.com/1025944 [ Linux ] external/wpt/pointerevents/pointerlock/pointerevent_pointermove_on_chorded_mouse_button_when_locked.html [ Pass Failure ]
+crbug.com/1026046 [ Mac10.10 ] compositing/contents-opaque/control-layer.html [ Pass Failure ]