[sheriff] Disable video reflection tests

Disable the following flaky tests:
 * compositing/video/video-reflection.html
 * compositing/reflections/load-video-in-reflection.html

TBR=schenney@chromium.org

Bug: 1084637, 1083362
Change-Id: Ic12cee8fb21917bcc76681399f59a24068dc0cf3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222520Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773156}

third_party/blink/web_tests/TestExpectations

@@ -5888,3 +5888,7 @@ crbug.com/1087077 external/wpt/html/semantics/forms/form-submission-0/form-doubl
 
 # Flaky timeout in the Debug build
 crbug.com/1085564 [ Debug ] virtual/threaded-no-composited-antialiasing/animations/interpolation/svg-x-interpolation.html [ Pass Timeout ]
+
+# Sheriff 2020-05-29
+crbug.com/1084637 compositing/video/video-reflection.html [ Pass Failure ]
+crbug.com/1083362 compositing/reflections/load-video-in-reflection.html [ Pass Failure ]