Correctly render iframes. Closes #100.

Code for detecting if browser enforced same origin policy for iframes
using different src protocols was not working correctly and an exception
was being thrown. This meant that some browsers were not able to
generate iframes for the examples.

share/docs.previews.js

@@ -68,8 +68,13 @@ var iframeEl = document.createElement('iframe');
 iframeEl.src = 'data:text/html,';
 bodyEl.appendChild(iframeEl);
 iframeEl.addEventListener('load', function() {
-  var support = { sameOriginDataUri: false };
-  if (this.contentDocument) support.sameOriginDataUri = true;
+  var support = { sameOriginDataUri: true };
+  try {
+    this.contentDocument;
+    if (!this.contentDocument) support.sameOriginDataUri = false;
+  } catch (e) {
+    support.sameOriginDataUri = false;
+  }
   this.parentNode.removeChild(this);
   // Loop through code textareas and render the code in iframes.
   forEach(bodyEl.getElementsByTagName('textarea'), function(codeEl, idx) {