Mark doubletap-to-jump-backwards-at-start.html timeout on Mac

TBR=beccahughes@chromium.org
NOTRY=true

Bug: 799137
Change-Id: Ia0d1557e9570e4b76263ce9ee386e95c6327220d
Reviewed-on: https://chromium-review.googlesource.com/851093
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527051}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3556,3 +3556,4 @@ crbug.com/798592 [ Win7 ] virtual/threaded/http/tests/devtools/tracing/timeline-
 # Sheriff failures 2018-01-03
 crbug.com/798680 external/wpt/pointerevents/pointerlock/pointerevent_pointerlock_supercedes_capture-manual.html [ Pass Timeout ]
 crbug.com/799127 [ Mac ] external/wpt/resource-timing/test_resource_timing.html [ Failure ]
+crbug.com/799137 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-to-jump-backwards-at-start.html [ Pass Timeout ]