Fix test failure about zero_size_with_child

A test about parent has zero width and height can not pass.

gentest/fixtures/YGDimensionTest.html

@@ -7,3 +7,9 @@
     <div style="width: 100px; height: 100px;"></div>
   </div>
 </div>
+
+<div id="zero_size_with_child">
+  <div style="width: 0px; height: 0px;">
+    <div style="width: 100px; height: 100px;"></div>
+  </div>
+</div>

gentest/gentest.js

@@ -411,6 +411,8 @@ function getDefaultStyleValue(style) {
     case 'bottom':
     case 'start':
     case 'end':
+    case 'width':
+    case 'height':
       return 'undefined';
   }
   var node = document.getElementById('default');