Mark fast/scroll-snap/snaps-after-scrollbar-scrolling.html as flaky/slow

Bug: 864887
Change-Id: I9210134ed670cd46026cd6f9d83a6336460ea140
Tbr: sunyunjia@chromium.org
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1141453Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575945}

third_party/WebKit/LayoutTests/SlowTests

@@ -399,3 +399,5 @@ crbug.com/838057 [ Linux ] virtual/outofblink-cors/http/tests/fetch/chromium/rel
 crbug.com/833215 external/wpt/accelerometer/Accelerometer.https.html [ Slow ]
 
 crbug.com/861670 [ Linux Mac ] virtual/gpu/fast/canvas/canvas-blending-image-over-gradient.html [ Slow ] 
+
+crbug.com/864887 [ Mac ] fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Slow ]

third_party/WebKit/LayoutTests/TestExpectations

@@ -4629,3 +4629,5 @@ crbug.com/860706 [ Mac ] virtual/gpu/fast/canvas/color-space/canvas-createImageB
 crbug.com/860706 [ Mac ] virtual/gpu/fast/canvas/canvas-blending-color-over-pattern.html [ Timeout Pass ]
 crbug.com/860706 [ Mac ] virtual/gpu/fast/canvas/canvas-blending-pattern-over-pattern.html [ Timeout Pass ]
 crbug.com/860706 [ Mac ] virtual/gpu/fast/canvas/canvas-shadow-source-in.html [ Timeout Pass ]
+
+crbug.com/864887 [ Linux ] fast/scroll-snap/snaps-after-scrollbar-scrolling.html [ Failure Pass ]