Ensure template helpers in trusted templates work.

lib/utils/templatize.html

@@ -503,7 +503,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         // Under strictTemplatePolicy, the templatized element must be owned
         // by a (trusted) Polymer element, indicated by existence of _methodHost;
         // e.g. for dom-if & dom-repeat in main document, _methodHost is null
-        if (Polymer.strictTemplatePolicy && !owner._methodHost) {
+        if (Polymer.strictTemplatePolicy && !findMethodHost(template)) {
           throw new Error('strictTemplatePolicy: template owner not trusted');
         }
         options = /** @type {!TemplatizeOptions} */(options || {});