remove unneeded protection code for extends.

src/lib/polymer-bootstrap.html

@@ -34,15 +34,10 @@
       // Polymer.Base is now chained to factory.prototype, and for IE10 compat
       // this may have resulted in a new prototype being created
       prototype = factory.prototype;
-      // native Custom Elements treats 'undefined' extends property
-      // as valued, the property must not exist to be ignored
-      // TODO(sjmiles): Custom Elements no longer has the above behavior
       var options = {
-        prototype: prototype
+        prototype: prototype,
+        extends: prototype.extends
       };
-      if (prototype.extends) {
-        options.extends = prototype.extends;
-      }
       Polymer.telemetry._registrate(prototype);
       document.registerElement(prototype.is, options);
       return factory;