sheriff: Update TestExpectations for expected css scroll diff flake

Marking external/wpt/css/css-scroll-snap/scroll-margin.html as flaky.
This was previously disabled for prefix virtual/threaded, but because
of a wpt change, we expect the test to flake for all variants.

TBR=majidvp@chromium.org

Bug: 964239
Change-Id: Iee686a55ff9142a81a495ee211b65cbb5a0aa508
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619833
Reviewed-by: Madeleine Barowsky <mbarowsky@chromium.org>
Commit-Queue: Madeleine Barowsky <mbarowsky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661359}
diff --git a/third_party/blink/web_tests/TestExpectations b/third_party/blink/web_tests/TestExpectations
index 0247fdd..a94f0ac9 100644
--- a/third_party/blink/web_tests/TestExpectations
+++ b/third_party/blink/web_tests/TestExpectations
@@ -5630,3 +5630,4 @@
 crbug.com/963739 [ Fuchsia ] synthetic_gestures/smooth-scroll-tiny-delta.html [ Pass Timeout ]
 crbug.com/964158 [ Linux ] external/wpt/payment-handler/change-payment-method.https.html [ Pass Crash ]
 crbug.com/899710 [ Linux ] virtual/threaded/http/tests/devtools/tracing/timeline-paint/timeline-paint-with-layout-invalidations.js [ Pass Timeout ]
+crbug.com/964239 external/wpt/css/css-scroll-snap/scroll-margin.html [ Pass Failure ]