fix(demo): fix centering of MDL icons

Some icons on the demo were not centered correctly, such as the custom
content tab's "plus" button.  Some conflicting styles were changing
the line-height.

Change-Id: I08c9bb01642346b1a3b1162f779300221ae23246

demo/demo.less

@@ -27,6 +27,12 @@
   font-family: 'Material Icons Round';
 }
 
+/* Conflicting styles between the icons font & the MDL buttons caused some
+ * icons not to be centered in their MDL button.  This fixes it. */
+.mdl-button .material-icons {
+  line-height: 36px;
+}
+
 /* This is a less mixin only, rather than a CSS class. MDL has an equivalent
  * class with the same name, which can be used in the application directly. */
 .hidden() {