Mark fast/scroll-behavior/smooth-scroll/keyboard-scroll.html slow

This test is consistently slow on Mac 10.13.

TBR=sahel@chromium.org
NOTRY=true

Bug: 898378
Change-Id: I09c144a06023918f6b4965409cecdb7743c095dc
Reviewed-on: https://chromium-review.googlesource.com/c/1297733Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602228}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5462,3 +5462,4 @@ crbug.com/898156 [ Linux Mac Win ] inspector-protocol/runtime/runtime-console-lo
 crbug.com/898214 [ Mac10.10 Mac10.11 ] external/wpt/html/dom/interfaces.https.html?exclude=(Document|Window|HTML.*) [ Failure ]
 crbug.com/898214 [ Mac10.11 ] virtual/outofblink-cors-ns/external/wpt/fetch/api/credentials/authentication-basic.any.html [ Failure ]
 crbug.com/898050 [ Mac Win ] fast/workers/taskqueue/basic.html [ Pass Timeout ]
+crbug.com/898378 [ Mac10.13 ] fast/scroll-behavior/smooth-scroll/keyboard-scroll.html [ Timeout ]