Skip 'custom-scrollbar-inactive-pseudo.html' layout tests.

Very flaky.

TBR=ojan@chromium.org

Bug: 789567
Change-Id: Ie457f0b626d3fe12af6f21b07fed9852c5902f13
Reviewed-on: https://chromium-review.googlesource.com/796890
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520137}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4064,6 +4064,9 @@ crbug.com/789139 [ Linux ] http/tests/devtools/sources/debugger/live-edit-no-rev
 crbug.com/789139 [ Linux ] virtual/mojo-loading/http/tests/devtools/sources/debugger/live-edit-no-reveal.js [ Pass Failure ]
 crbug.com/785179 [ Mac10.11 ] http/tests/devtools/console/console-viewport-stick-to-bottom.js [ Pass Failure ]
 # Sheriff failures 2017-11-29
+crbug.com/789567 [ Mac ] virtual/rootlayerscrolls/scrollbars/custom-scrollbar-inactive-pseudo.html [ Failure Pass ]
+crbug.com/789567 [ Mac ] virtual/rootlayerscrolls/scrollbars/custom-scrollbar-adjust-on-inactive-pseudo.html [ Failure Pass ]
+crbug.com/789567 [ Mac ] virtual/prefer_compositing_to_lcd_text/scrollbars/custom-scrollbar-inactive-pseudo.html [ Failure Pass ]
 crbug.com/789478 [ Mac ] virtual/prefer_compositing_to_lcd_text/scrollbars/custom-scrollbar-adjust-on-inactive-pseudo.html [ Pass Failure ]
 crbug.com/789485 [ Linux ] virtual/mouseevent_fractional/fast/events/touch/gesture/gesture-tap-mouse-events-between-frames.html [ Pass Failure ]
 crbug.com/789533 virtual/gpu/fast/canvas/OffscreenCanvas-2d-imageSmoothing.html [ Pass Failure ]