Mark flaky layout test

external/wpt/html/semantics/forms/constraints/form-validation-reportValidity.html

TBR=robertma,foolip

Bug: 1041973
Change-Id: Id554bd1ea92db462040836d3c74ec985c2d68627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001577Reviewed-by: Roberto Carrillo <robertocn@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731723}

third_party/blink/web_tests/TestExpectations

@@ -6422,4 +6422,6 @@ crbug.com/963183 http/tests/devtools/sources/debugger-breakpoints/disable-breakp
 
 crbug.com/836300 fast/css3-text/css3-text-decoration/text-decoration-skip-ink-links.html [ Pass Failure ]
 
-
+# Sheriff 2020-01-14
+crbug.com/1041973 external/wpt/html/semantics/forms/constraints/form-validation-reportValidity.html [ Pass Failure ]
+crbug.com/1041973 virtual/web-components-v0-disabled/external/wpt/html/semantics/forms/constraints/form-validation-reportValidity.html [ Pass Failure ]