blink: Mark events_referenceSpace_reset_inline.https.html as flaky on Mac10.12

https://ci.chromium.org/p/chromium/builders/ci/Mac10.12%20Tests
https://ci.chromium.org/p/chromium/builders/ci/Mac10.12%20Tests/31990

TBR=alcooper@chromium.org

Bug: 1053903
Change-Id: Idbd00ecfe63d58649969714912fffa8d083fefe4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062275
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742560}

third_party/blink/web_tests/TestExpectations

@@ -6626,3 +6626,6 @@ crbug.com/1051774 [ Fuchsia ] http/tests/plugins/navigator-plugins-in-cross-orig
 crbug.com/1051133 [ Fuchsia ] http/tests/canvas/philip/tests/security.pattern.canvas.fillStyle.html [ Crash Timeout Pass ]
 crbug.com/1051131 [ Fuchsia ] http/tests/appcache/abort-cache-onchecking-resource-404.html [ Timeout Pass ]
 crbug.com/848799 [ Win10 ] http/tests/devtools/coverage/multiple-instances-merge.js [ Pass Timeout ]
+
+# Sheriff 2020-02-19
+crbug.com/1053903 [ Mac10.12 ] external/wpt/webxr/events_referenceSpace_reset_inline.https.html [ Pass Failure ]