change default value of position styles

Summary: Default value for positions should be undefined not 0px. Fixing this leads to more correct tests.

Reviewed By: gkassabli

Differential Revision: D4153329

fbshipit-source-id: d0f194f9c47eac93d3815ec7e55568a1016bc7fe

gentest/fixtures/CSSLayoutAbsolutePositionTest.html

@@ -15,7 +15,7 @@
 </div>
 
 <div id="do_not_clamp_height_of_absolute_node_to_height_of_its_overflow_hidden_parent" style="height: 50px; width: 50px; overflow: hidden; flex-direction: row;">
-  <div style="position: absolute; start: 0; top: 0;">
+  <div style="position: absolute; start: 0px; top: 0px;">
     <div style="width: 100px; height: 100px;"></div>
   </div>
 </div>

gentest/gentest.js

@@ -159,10 +159,10 @@ function checkDefaultValues() {
     {style:'overflow', value:'visible'},
     {style:'flex-grow', value:'0'},
     {style:'flex-shrink', value:'0'},
-    {style:'left', value:'0px'},
-    {style:'top', value:'0px'},
-    {style:'right', value:'0px'},
-    {style:'bottom', value:'0px'},
+    {style:'left', value:'undefined'},
+    {style:'top', value:'undefined'},
+    {style:'right', value:'undefined'},
+    {style:'bottom', value:'undefined'},
   ].forEach(function(item) {
     assert(item.value === getDefaultStyleValue(item.style),
         item.style + ' should be ' + item.value);
@@ -415,6 +415,15 @@ function getDefaultStyleValue(style) {
   if (style == 'position') {
     return 'relative';
   }
+  switch (style) {
+    case 'left':
+    case 'top':
+    case 'right':
+    case 'bottom':
+    case 'start':
+    case 'end':
+      return 'undefined';
+  }
   var node = document.getElementById('default');
   return getComputedStyle(node, null).getPropertyValue(style);
 }