cros: Minor style change in wallpaper picker to match KSV

This is a workaround. The bug should keep open until a better solution
is found. Verified on both eve (high PPI) and paine (low PPI).

Also, "margin-bottom" is a no-op so it's removed.

Bug: 889655
Change-Id: I424d1c15ba2ee15a62b098f5bffc28037707ef97
Reviewed-on: https://chromium-review.googlesource.com/1247193Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594905}

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

@@ -435,8 +435,7 @@ body.v2 {
 
 .v2 #categories-list > li {
   border-top: unset;
-  height: 32px;
-  margin-bottom: 8px;
+  height: 31px;
   margin-top: 8px;
   pointer-events: auto;
 }