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This CL adds CSS for .separator which is a grey separator line
between controls on a row (e.g. settings-box). The intent is to replace
.secondary-action (a TODO has been added to that effect). This initial
CL changes the separator lines seen in the basic main page of MD
settings.

(a step to fix, but not complete fix).
BUG=714320
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2847583002
Cr-Commit-Position: refs/heads/master@{#468471}