[CrOS Wallpapers] Allow all wallpaper categories to fit.

We are adding 3 new wallpaper apps, but this causes the "My Images"
category to not be shown. This CL shrinks the margin between category
list items so that all wallpaper categories in the list will be shown
upon opening the app.

https://screenshot.googleplex.com/h4QERzczpze5qK9

Fixed: 1129959
Change-Id: I631e28b6123804461794664d6ae37cd090ce5268
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419003Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808535}

chrome/browser/resources/chromeos/wallpaper_manager/css/wallpaper_manager.css

@@ -68,7 +68,7 @@ body {
   color: #777;
   display: -webkit-box;
   height: 31px;
-  margin-top: 8px;
+  margin-top: 4px;
   overflow: hidden;
   pointer-events: auto;
   position: relative;