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

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

Index: chrome/browser/resources/settings/settings_ui/settings_ui.html
diff --git a/chrome/browser/resources/settings/settings_ui/settings_ui.html b/chrome/browser/resources/settings/settings_ui/settings_ui.html
index 30e7b38f4af5b462cadb9a2dbc8a01d6ad636be7..35301acca3e10ce042f9b92af0a8331b99ea812d 100644
--- a/chrome/browser/resources/settings/settings_ui/settings_ui.html
+++ b/chrome/browser/resources/settings/settings_ui/settings_ui.html
@@ -35,6 +35,12 @@
         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);
       }