[ScrollSnap] Update test flaky type

There have been new flaky type reported on Mac recently and the test on
Win seems to be normal:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests&tests=virtual%2Fthreaded%2Ffast%2Fscroll-snap%2Fsnaps-after-keyboard-scrolling.html

Bug: 1056957
Change-Id: I2f6afefbee94ecae09ca09503df55afcc12d2c97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463323Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815596}

third_party/blink/web_tests/TestExpectations

@@ -5893,8 +5893,7 @@ crbug.com/1041830 http/tests/devtools/tracing/timeline-js/timeline-js-line-level
 
 # Enable scroll-snap tests on impl thread
 # These are currently failing on Mac which needs more investigation, snap-scrolls-visual-viewport seems flaky
-crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Timeout ]
-crbug.com/878878 [ Win ] virtual/threaded/fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Timeout ]
+crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Timeout Failure ]
 crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Pass Timeout Failure ]
 crbug.com/878878 [ Win ] virtual/threaded/fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Pass Timeout Failure ]