[MD settings] set margins on change download location button

This CL sets the padding/margin on buttons so that there is 12px on either
side of the button label text (per UI design).

BUG=714352
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2870773002
Cr-Commit-Position: refs/heads/master@{#470650}
2 files changed