removed isTopWindow function

lib/single-file/frame-tree.js

@@ -28,7 +28,7 @@ this.frameTree = this.frameTree || (() => {
 	const INIT_RESPONSE_MESSAGE = "initResponse";
 	const TIMEOUT_INIT_REQUEST_MESSAGE = 500;
 	const TOP_WINDOW_ID = "0";
-	const TOP_WINDOW = isTopWindow(window);
+	const TOP_WINDOW = window == top;
 
 	let sessions = new Map(), windowId;
 
@@ -174,8 +174,4 @@ this.frameTree = this.frameTree || (() => {
 		}
 	}
 
-	function isTopWindow(win) {
-		return win == top;
-	}
-
 })();