Mark external/wpt/mediacapture-fromelement/capture.html flaky

BUG=1142877
TBR=poromov@chromium.org
NOTRY=TRUE

Change-Id: Ib657fd85f7bbe1daf8fd7368c856dda60db018b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502783Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821292}

third_party/blink/web_tests/TestExpectations

@@ -6299,3 +6299,6 @@ crbug.com/1140329 virtual/threaded/external/wpt/web-animations/timing-model/anim
 
 #Sheriff 2020-10-21
 crbug.com/1141206 scrollbars/overflow-scrollbar-combinations.html [ Pass Failure ]
+
+#Sheriff 2020-10-26
+crbug.com/1142877 external/wpt/mediacapture-fromelement/capture.html [ Pass Failure ]
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