Mark test media/controls-styling-strict.html as flaky

The test has been flaky on several bots:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests&tests=media%2Fcontrols-styling-strict.html

TBR=mlamouri@google.com
NOTRY=TRUE

Bug: 1083605
Change-Id: Ia0736180fc02f8e258f88d2566a03fe0dc293cdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437915Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811850}

third_party/blink/web_tests/TestExpectations

@@ -6734,4 +6734,5 @@ crbug.com/1057060 virtual/scroll-unification/fast/scrolling/scrollbars/mouse-aut
 crbug.com/1045599 fast/css-grid-layout/flex-content-sized-columns-resize.html [ Pass Failure Timeout ]
 
 # Sheriff 2020-09-29
-crbug.com/1133342 external/wpt/html/cross-origin-opener-policy/reporting/access-reporting/reporting-observer.html [ Pass Failure Timeout ]
+crbug.com/1133342 external/wpt/html/cross-origin-opener-policy/reporting/access-reporting/reporting-observer.html [ Pass Failure Timeout ]
\ No newline at end of file
+crbug.com/1083605 media/controls-styling-strict.html [ Pass Failure ]