Dark Mode: update history time gap separator color

R=dpapad@chromium.org
BUG=883049

Change-Id: I23f3973721ae92fbb65d6e346e35a6e48a464615
Reviewed-on: https://chromium-review.googlesource.com/c/1416818
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623553}
1 file changed