Settings Rewrite: Make subpage scrolling work.

chrome/browser/resources/settings/settings_main/settings_main.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
-:host {
-  bottom: 0;
-  color: black;
-  display: block;
-  left: 0;
-  position: absolute;
-  right: 0;
-  top: 0;
-}
-
-#mainContainer {
+#pageContainer {
   box-sizing: border-box;
   height: 100%;
   overflow: auto;
   padding: 34px 16px;
 }
 
+#pageContainer.expanded {
+  padding: 0 16px;
+}
+
 ::content core-icon-button {
   position: absolute;
 }