Use StyleNode in StockMenu

StockMenu was creating a container for the sole purpose of applying style to
PopupMenu. Now we just use a StyleNode.

Also, I've reverted the change to make box-sizing default to border-box. It
turns out that CL wasn't effective because we didn't use the initialBoxSizing
function to initialize box sizing. I've made us use initialBoxSizing but switch
the default back to content-box because actually using border-box breaks a
bunch of stuff.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1024083003

examples/stocks-fn/lib/stock_menu.dart

@@ -17,9 +17,7 @@ class StockMenu extends Component {
   StockMenu({Object key, this.controller}) : super(key: key);
 
   Node build() {
-    return new Container(
-      style: _style,
-      children: [
+    return new StyleNode(
       new PopupMenu(
         controller: controller,
         items: [
@@ -27,8 +25,8 @@ class StockMenu extends Component {
           [new Text('Remove stock')],
           [new Text('Help & feeback')],
         ],
-          level: 4)
-      ]
+        level: 4),
+        _style
     );
   }
 }