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According to the mock, separators should go between sections, not items.
This CL fixes the style according to the mock.

Given that this changes the UI of the app, it might be better to check in
this after the M64 branch point.

Before: https://drive.google.com/a/google.com/file/d/1cosHV0dLNdIraqZCQhE0M0La9b6fDn6B/view?usp=sharing
After: https://drive.google.com/a/google.com/file/d/1s_ZtlUMztnkr-QdU1-04ssxw-m2iLXHH/view?usp=sharing

Bug: 786616
Change-Id: If4975398b7fd0da0a0884ee4b009c3098f05de42
Reviewed-on: https://chromium-review.googlesource.com/783817
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521488}