fix cross-origin iframe and no parent debugbar (php-debugbar#651)

* fix `cross-origin` iframe and no parent debugbar

* fix iframe flag default value

Author: erikn69

src/DebugBar/Resources/debugbar.js

@@ -425,7 +425,11 @@ if (typeof(PhpDebugBar) == 'undefined') {
             this.activeDatasetId = null;
             this.datesetTitleFormater = new DatasetTitleFormater(this);
             this.options.bodyMarginBottomHeight = parseInt($('body').css('margin-bottom'));
-            this.isIframe = window.self !== window.top;
+            try {
+                this.isIframe = window.self !== window.top && window.top.phpdebugbar;
+            } catch (error) {
+                this.isIframe = false;
+            }
             this.registerResizeHandler();
         },