[FIX] website editor clear()

Apparently in some unclear situations, using delete on the dataset blows
up (issue appears in both Safari and phantomjs) failing the test.

Only try the deletion if the item is actually present in the dataset,
and wrap it in a try/except to handle it blowing up

addons/website/static/src/js/website.editor.js

@@ -2512,7 +2512,14 @@ var VideoDialog = Widget.extend({
         this.media = $iframe[0];
     },
     clear: function () {
-        delete this.media.dataset.src;
+        if (this.media.dataset.src) {
+            try {
+                delete this.media.dataset.src;
+            } catch(e) {
+                this.media.dataset.src = undefined;
+            }
+        }
+
         this.media.className = this.media.className.replace(/(^|\s)media_iframe_video(\s|$)/g, ' ');
     },
 });