[Sheriff] Disable external/wpt/pointerevents/pointerevent_coalesced_events_attributes.html.

This test is flaky on
https://ci.chromium.org/p/chromium/builders/ci/Mac10.14%20Tests/2309.

BUG=1059645
TBR=tkent@chromium.org

Change-Id: I3e96afa16ec0f7781aac489dffe744b3066fdf91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094418Reviewed-by: Maggie Cai <mxcai@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748114}

third_party/blink/web_tests/TestExpectations

@@ -6771,3 +6771,6 @@ crbug.com/1059322 [ Mac ] fast/forms/file/file-input-click.html [ Pass Failure ]
 crbug.com/1059322 [ Mac ] fast/forms/autofocus-focus-only-once.html [ Pass Failure ]
 crbug.com/1059322 [ Mac ] virtual/form-controls-refresh-disabled/fast/forms/autofocus-focus-only-once.html [ Pass Failure ]
 crbug.com/1058244 [ Mac ] virtual/threaded-prefer-compositing/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.html [ Pass Failure ]
+
+# Sheriff 2020-03-08
+crbug.com/1059645 [ Mac10.14 ] external/wpt/pointerevents/pointerevent_coalesced_events_attributes.html [ Pass Failure ]