[ScrollSnap] Mark test as flaky

The test fast/scroll-snap/snaps-after-keyboard-scrolling.html
started to be flaky on fast path after being enabled on it in
crbug.com/878878.

Bug:878878, 1056957

Change-Id: I506f9a63ac10c059b7e9e30da29c36826a2d6bde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084110Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746393}

third_party/blink/web_tests/TestExpectations

@@ -6277,7 +6277,8 @@ crbug.com/1035309 http/tests/devtools/throttling/mobile-throttling.js [ Pass Fai
 
 # 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 [ Timeout ]
+crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Timeout ]
+crbug.com/878878 [ Win10 ] virtual/threaded/fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Timeout ]
 crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Timeout ]
 crbug.com/878878 [ Mac ] virtual/threaded/fast/scroll-snap/snaps-for-different-key-granularity.html [ Timeout ]