MD Settings: Fix side navigation bar background color for RTL.

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 ef34d4535f2f68ef54f2a1be82af5831481f676e..9a8d8d801dbe98bcfdfa3a562a84893e2d33b5af 100644
--- a/chrome/browser/resources/settings/settings_ui/settings_ui.html
+++ b/chrome/browser/resources/settings/settings_ui/settings_ui.html
@@ -46,6 +46,10 @@
           background-color: var(--settings-background-color);
           margin-top: 8px;
         };
+        --paper-drawer-panel-right-drawer-container: {
+          background-color: var(--settings-background-color);

[michaelpg, 2016/03/23 00:18:32]

it would be nice if we didn't have to duplicate this

I guess we can't comma-separate the mixins like we can with toplevel CSS rules,
but wdyt about creating a separate common mixin to @apply in each of these
mixins?

or would that be terrible in itself?

[dpapad, 2016/03/23 00:40:18]

I am not very familiar with CSS mixins. Can you use @apply within a mixin? Also,
if I understand your suggestion it would look something as follows,

--side-nav-background: {
  background-color: var(--settings-background-color);
}
--paper-drawer-panel-left-drawer-container: {
  @apply(--side-nav-background);
}
--paper-drawer-panel-right-drawer-container: {
  @apply(--side-nav-background);
}

So now instead of repeating the CSS variable twice, we have an extra level of
indirection, so we repeat the new mixin twice. Is that any better (or am I
misunderstanding the suggestion)?

+          margin-top: 8px;
+        };
         position: relative;
       }