Mark some threaded-prefer-compositing/fast/scroll-behavior tests as flaky

TBR=wangxianzhu@chromium.org

Bug: 1048113
Change-Id: Ib5eb4bc816f02cfdef0b7968cc2c8c92cd1601f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2034525Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737743}

third_party/blink/web_tests/TestExpectations

@@ -6454,3 +6454,8 @@ crbug.com/1047126 [ Mac10.14 ] external/wpt/css/css-color/color-initial-canvaste
 
 # Sheriff 2020-01-31
 crbug.com/1047760 [ Win ] http/tests/devtools/screen-orientation-override.js [ Pass Timeout ]
+
+# Sheriff 2020-02-03
+crbug.com/1048113 [ Debug ] virtual/threaded-prefer-compositing/fast/scroll-behavior/main-frame-interrupted-scroll.html [ Pass Failure ]
+crbug.com/1048113 [ Debug ] virtual/threaded-prefer-compositing/fast/scroll-behavior/main-frame-scrollLeft.html [ Pass Failure ]
+crbug.com/1048113 [ Debug ] virtual/threaded-prefer-compositing/fast/scroll-behavior/subframe-interrupted-scroll.html [ Pass Failure ]