Mark as http/tests/devtools/coverage/coverage-view-unused.js as flaky on Mac/Win.

Bug: 1038656
Change-Id: Id2cfb02d6ccaaddab0e641b48b9d7e2857014f0c
No-Try: true
TBR: janscheffler@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985764Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727968}

third_party/blink/web_tests/TestExpectations

@@ -6150,3 +6150,7 @@ crbug.com/1038139 [ Win ] virtual/gpu-rasterization/images/2-comp.html [ Pass Fa
 # Sheriff 2019-12-30
 crbug.com/1037798 [ Mac ] virtual/gpu-rasterization/images/exif-orientation-image-document.html [ Pass Failure ]
 crbug.com/1038354 fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Pass Failure ]
+
+# Sheriff 2020-01-02
+crbug.com/1038656 [ Mac ] http/tests/devtools/coverage/coverage-view-unused.js [ Pass Failure ]
+crbug.com/1038656 [ Win ] http/tests/devtools/coverage/coverage-view-unused.js [ Pass Failure ]