MD Settings: Introduce CrScrollableBehavior

ui/webui/resources/cr_elements/shared_style_css.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 
 <!-- Common styles for Material Design WebUI. Included directly in
   settings_shared_css.html. -->
 <dom-module id="cr-shared-style">
   <template>
     <style>
       /* Chrome spinners should be blue. */
       paper-spinner {
@@ skipping 23 matching lines @@
 
       .action-button,
       .cancel-button {
         font-weight: 500;
       }
 
       [actionable] {
         @apply(--cr-actionable);
       }
 
+      [scrollable] {
+        border-color: transparent;
+        border-style: solid;
+        border-width: 1px 0;
+        overflow-y: auto;
+      }
+      [scrollable].is-scrolled {
+        border-top-color: var(--google-grey-300);
+      }
+      [scrollable].can-scroll:not(.scrolled-to-bottom) {
+        border-bottom-color: var(--google-grey-300);
+      }
+      [scrollable] :focus {
+        @apply(--cr-list-item-focus);
+        @apply(--cr-selectable-focus);
+      }
+      [scrollable] iron-list > * {
+        @apply(--cr-actionable);
+      }
+
       [selectable] :focus {
         @apply(--cr-selectable-focus);
       }
       [selectable] > * {
         @apply(--cr-actionable);
       }
     </style>
   </template>
 </dom-module>