Disable flaky horizontal-smooth-scroll-in-rtl.html.

TBR=sunyunjia@chromium.org

Bug: 664858
Change-Id: I8d7332fdb6755347177fcf7e736e0d7f4d6e533d
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/827016Reviewed-by: Henrik Grunell <grunell@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524054}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2748,6 +2748,7 @@ crbug.com/702837 fast/text/aat-morx.html [ Skip ]
 crbug.com/688670 media/track/track-cue-rendering-after-controls-removed.html [ Pass Failure ]
 
 crbug.com/664858 virtual/threaded/fast/scroll-behavior/overflow-scroll-animates.html [ Skip ]
+crbug.com/664858 virtual/threaded/fast/scroll-behavior/smooth-scroll/horizontal-smooth-scroll-in-rtl.html [ Skip ]
 crbug.com/664858 virtual/threaded/fast/scroll-behavior/smooth-scroll/main-thread-scrolling-reason-added.html [ Skip ]
 crbug.com/664858 virtual/threaded/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-anchors.html [ Skip ]
 crbug.com/664858 virtual/threaded/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-vertical-rl-anchors.html [ Skip ]