[MD settings] allow horizontal scroll bar in settings content area

chrome/browser/resources/settings/settings_ui/settings_ui.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/app-layout/app-drawer/app-drawer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-header-panel/paper-header-panel.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="/direction_delegate.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/icons.html">
 <link rel="import" href="/settings_main/settings_main.html">
 <link rel="import" href="/settings_menu/settings_menu.html">
 <link rel="import" href="/settings_page/settings_router.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-ui">
   <template>
     <style include="settings-shared">
       :host {
         @apply(--layout-fit);
+        --paper-header-panel-waterfall-container: {
+          overflow-x: auto;

[tommycli, 2016/06/28 23:29:27]

Since not immediately obvious, I just recommend a comment explaining that it
restores the horiz scroll bar and for whatever reason paper-foo disables it.

[dschuyler, 2016/06/28 23:34:31]

Done.

+        };
         -webkit-user-select: none;
         background-color: var(--settings-background-color);
         display: flex;
         flex-direction: column;
       }
 
       paper-header-panel {
         --paper-header-panel-shadow: {
           z-index: 2;
         };
@@ skipping 70 matching lines @@
         </settings-menu>
       </div>
     </app-drawer>
     <paper-header-panel mode="waterfall">
       <settings-main prefs="{{prefs}}" current-route="{{currentRoute}}">
       </settings-main>
     </paper-header-panel>
   </template>
   <script src="settings_ui.js"></script>
 </dom-module>