[Sheriff] Mark fast/scroll-snap/snaps-after-scrollbar-scrolling.html as flaky

Bug: 1038354
Change-Id: I03c38410a2bc277cd9a2083c26f32f508c7510fa
Tbr: xiaochengh@chromium.org
NoTry: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1984461Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727776}

third_party/blink/web_tests/TestExpectations

@@ -6156,3 +6156,4 @@ 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 ]