[OsSettingsLanguages] Update dialog button container paddings

Because of the presence of iron-list, there's a horizontal line
separating the buttons from the list, making the padding difference
look very obvious. Update all to 20px. Spec has the padding left and
right at 24px but current dialog body padding is 20px, so sticking
to that.

Spec: http://go/cros-lang-settings-figma
Current view: http://screen/4EAq4TaqjwEyuTq

Bug: 1113439
Change-Id: Ief84905331ba8965854b34252308161cd3bf6eef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437069
Commit-Queue: My Nguyen <myy@chromium.org>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811631}

chrome/browser/resources/settings/chromeos/os_languages_page/shared_style.html

@@ -21,6 +21,13 @@
         border-radius: 4px;
         margin-inline-end: 20px;
       }
+
+      cr-dialog [slot=button-container] {
+        padding-bottom: 20px;
+        padding-inline-end: 20px;
+        padding-inline-start: 20px;
+        padding-top: 20px;
+      }
     </style>
   </template>
 </dom-module>
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