Fix spacing of text/button in settings sync promo

Add horizontal margin to the button.

Bug: 933755
Change-Id: I74423dbf88a69c8577a47a29502a3b75f9dc4cf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529285
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642490}