MD History: Reduce space between cards

This changes the spacing between cards from 20px to 16px.

BUG=670049
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2538333002
Cr-Commit-Position: refs/heads/master@{#435543}
2 files changed