[Sheriff] Mark test fast/scroll-snap/snaps-after-keyboard-scrolling-rtl.html flaky on all platforms

Test started to be flaky on Mac as well.

TBR=majidvp@chromium.org
NOTRY=true

Bug: 986018
Change-Id: Ic896095e10af871eb7734f3730cfc58317475890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712513Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679529}

third_party/blink/web_tests/TestExpectations

@@ -6305,4 +6305,4 @@ crbug.com/985869 [ Linux Win7 ] virtual/blink-cors/http/tests/security/contentSe
 
 # Sheriff 2019-07-20
 crbug.com/986019 [ Linux Win ] animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]
-crbug.com/986018 [ Linux Win ] fast/scroll-snap/snaps-after-keyboard-scrolling-rtl.html [ Pass Failure ]
+crbug.com/986018 fast/scroll-snap/snaps-after-keyboard-scrolling-rtl.html [ Pass Failure ]