MD Settings: Fix regression causing overscroll to not shrink.

chrome/browser/resources/settings/settings_ui/settings_ui.html

 <link rel="import" href="chrome://resources/cr_elements/cr_drawer/cr_drawer.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout.html">
 <link rel="import" href="../direction_delegate.html">
 <link rel="import" href="../global_scroll_target_behavior.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../settings_main/settings_main.html">
@@ skipping 41 matching lines @@
         --iron-icon-fill-color: white;
         background-color: var(--settings-title-bar-background-color);
         color: white;
         min-height: 56px;
         z-index: 2;
       }
 
       #container {
         flex: 1;
         overflow: overlay;
+        position: relative;
       }
 
       #dropShadow {
         /* TODO(dpapad): This box-shadow is repeated in Settings, History and
            Downloads. Define a CSS variable instead and re-use. */
         box-shadow: inset 0 5px 6px -3px rgba(0, 0, 0, 0.4);
         height: 6px;
         left: 0;
         opacity: 0;
         pointer-events: none;
@@ skipping 38 matching lines @@
       <settings-main id="main" prefs="{{prefs}}"
           toolbar-spinner-active="{{toolbarSpinnerActive_}}"
           page-visibility="[[pageVisibility_]]"
           show-android-apps="[[showAndroidApps_]]"
           advanced-toggle-expanded="{{advancedOpened_}}">
       </settings-main>
     </div>
   </template>
   <script src="settings_ui.js"></script>
 </dom-module>