MD WebUI: Fix Action/Cancel button padding in MD History/Bookmarks

This fixes an issue where action/cancel buttons in MD History/Bookmarks
(and possibly other pages) had no vertical padding, after the default
paper-button padding was disabled in crrev.com/466545.

BUG=703998
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2837763002
Cr-Commit-Position: refs/heads/master@{#467238}
1 file changed