Mark paint/invalidation/table/cached-69296.html as flaky on Mac

TBR=wangxianzhu@chromium.org

Bug: 898485
Change-Id: Ib0adea0db6371d32aaab5e8714246efaaca15ed4
Reviewed-on: https://chromium-review.googlesource.com/c/1297428Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602318}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5462,3 +5462,4 @@ crbug.com/898378 [ Mac10.13 ] fast/scroll-behavior/smooth-scroll/keyboard-scroll
 # Sheriff 2018-10-24
 crbug.com/898394 [ Linux ] virtual/android/url-bar/bottom-and-top-fixed-sticks-to-top.html [ Failure ]
 crbug.com/898378 [ Mac10.13 ] virtual/scroll_customization/fast/scroll-behavior/smooth-scroll/keyboard-scroll.html [ Timeout ]
+crbug.com/898485 [ Mac ] paint/invalidation/table/cached-69296.html [ Failure Pass ]