Mark 5 test in external/.../counter-styles-3/ flaky

Tests:
symbols-function-invalid.html
system-additive-invalid.html
system-fixed-invalid.html
system-numeric-invalid.html
system-symbolic-invalid.htm

also flaky on Linux and Win

flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=external%2Fwpt%2Fcss%2Fvendor-imports%2Fmozilla%2Fmozilla-central-reftests%2Fcounter-styles-3%2Fsystem

No-Try: True
TBR: eirage@chromioum.org
Bug: 935689
Change-Id: Ib6724b44713a9a8cb7370adf781d948aa99615e2
Reviewed-on: https://chromium-review.googlesource.com/c/1495742
Commit-Queue: Ella Ge <eirage@chromium.org>
Reviewed-by: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636573}

third_party/blink/web_tests/TestExpectations

@@ -5904,12 +5904,12 @@ crbug.com/935027 http/tests/devtools/animation/animation-transition-setTiming-cr
 # Sheriff 2019-02-25
 crbug.com/935587 [ Mac ] virtual/video-surface-layer/media/stable/video-object-fit-stable.html [ Failure Pass ]
 crbug.com/935638 fast/dom/rtl-scroll-to-leftmost-and-resize.html [ Timeout Pass ]
-crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/symbols-function-invalid.html [ Failure ]
-crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-additive-invalid.html [ Failure ]
-crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-fixed-invalid.html [ Failure ]
-crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-numeric-invalid.html [ Failure ]
-crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-symbolic-invalid.html [ Failure ]
-crbug.com/935690 [ Mac ] virtual/video-surface-layer/media/video-played-ranges-1.html [ Failure ]
+crbug.com/935689 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/symbols-function-invalid.html [ Failure Pass ]
+crbug.com/935689 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-additive-invalid.html [ Failure Pass ]
+crbug.com/935689 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-fixed-invalid.html [ Failure Pass ]
+crbug.com/935689 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-numeric-invalid.html [ Failure Pass ]
+crbug.com/935689 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-symbolic-invalid.html [ Failure Pass ]
+crbug.com/935690 virtual/video-surface-layer/media/video-played-ranges-1.html [ Failure ]
 
 # Sheriff 2019-02-26
 crbug.com/935689 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/counter-styles-3/system-alphabetic-invalid.html [ Failure ]