MD Settings: make the background color of the collapsed nav drawer white

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-drawer-panel/paper-drawer-panel.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="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-toolbar/paper-toolbar.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://md-settings/direction_delegate.html">
 <link rel="import" href="chrome://md-settings/i18n_setup.html">
@@ skipping 17 matching lines @@
         @apply(--layout-center);
         --paper-drawer-panel-left-drawer-container: {
           background-color: var(--settings-background-color);
         };
         --paper-drawer-panel-right-drawer-container: {
           background-color: var(--settings-background-color);
         };
         position: relative;
       }
 
+      paper-drawer-panel[narrow] {
+        --paper-drawer-panel-left-drawer-container: {
+          background-color: white;

[dschuyler, 2016/05/03 00:36:12]

optional: maybe 
background-color: var(--settings-drawer-color);
where --settings-drawer-color is white? 
It amounts to the same thing, but it would help
keep the color definitions in the same place.
(That place being settings_shared_css.html right
now, but may move later).

[Dan Beam, 2016/05/03 00:37:10]

we shouldn't be adding variables if they're only used once.

i'll add a variable if this is needed anywhere else in settings or in other UIs
(i.e. history), which is likely.

+        };
+      }
+
       paper-icon-button {
         --iron-icon-fill-color: var(--settings-title-bar-color);
       }
 
       paper-icon-button[suffix] {
         --iron-icon-fill-color: var(--settings-title-search-color);
         height: 32px; /* After padding, the icon size is 16px. */
         width: 32px;
       }
 
@@ skipping 96 matching lines @@
             </paper-input>
           </div>
         </paper-toolbar>
         <settings-main prefs="{{prefs}}" current-route="{{currentRoute}}">
         </paper-icon-button>
       </paper-header-panel>
     </paper-drawer-panel>
   </template>
   <script src="settings_ui.js"></script>
 </dom-module>