make sure sessiosn is global (fix #945)

src/single-file/processors/frame-tree/content/content-frame-tree.js

@@ -64,7 +64,10 @@ const top = globalThis.top;
 const MessageChannel = globalThis.MessageChannel;
 const document = globalThis.document;
 
-const sessions = new Map();
+let sessions = globalThis.sessions;
+if (!sessions) {
+	sessions = globalThis.sessions = new Map();
+}
 let windowId;
 if (TOP_WINDOW) {
 	windowId = TOP_WINDOW_ID;