NTP Promos: Add horizontal padding for narrow windows

In the two side-by-side promos case, as the browser window narrows,
the promos will currently bump into the edge of the window before
wrapping into a column arrangement instead.  Supply a bit of padding
so wrapping happens earlier.

https://screenshot.googleplex.com/5QUBrCLBR34t6CL

Bug: 432494578
Change-Id: I611baf117b59611d2f0bc1a4c08658a501af08da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6885242
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506646}