Mark fast/spatial-navigation/snav-two-elements-one-line.html flaky

TBR=thestig@chromium.org
NOTRY=true

Bug: 836275
Change-Id: Idff1c4a5a600193bb8b3803ce4bcbbf205e5ccb7
Reviewed-on: https://chromium-review.googlesource.com/1026083Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553157}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4117,3 +4117,4 @@ crbug.com/836185 [ Mac10.11 Debug ] external/wpt/web-animations/animation-model/
 crbug.com/836185 [ Mac10.11 Debug ] external/wpt/web-animations/animation-model/animation-types/interpolation-per-property.html [ Crash ]
 crbug.com/836241 [ Linux ] virtual/android/fullscreen/video-controls-timeline.html [ Pass Failure ]
 crbug.com/836242 [ Linux Mac Win ] fast/loader/reload-zero-byte-plugin.html [ Pass Failure ]
+crbug.com/836275 [ Linux Mac ] fast/spatial-navigation/snav-two-elements-one-line.html [ Pass Failure ]