core: fix main session OOPIF checks for devtools (#12533)

lighthouse-core/lib/network-recorder.js

@@ -21,6 +21,8 @@ class NetworkRecorder extends EventEmitter {
     this._records = [];
     /** @type {Map<string, NetworkRequest>} */
     this._recordsById = new Map();
+    /** @type {string|null|undefined} */
+    this._mainSessionId = null;
   }
 
   /**
@@ -205,6 +207,20 @@ class NetworkRecorder extends EventEmitter {
    * @return {NetworkRequest|undefined}
    */
   _findRealRequestAndSetSession(requestId, sessionId) {
+    // The very first sessionId processed is always the main sessionId. In all but DevTools,
+    // this sessionId is undefined. However, in DevTools the main Lighthouse protocol connection
+    // does send events with sessionId set to a string, because of how DevTools routes the protocol
+    // to Lighthouse.
+    // Many places in Lighthouse use `record.sessionId === undefined` to mean that the session is not
+    // an OOPIF. To maintain this property, we intercept sessionId here and set it to undefined if
+    // it matches the first value seen.
+    if (this._mainSessionId === null) {
+      this._mainSessionId = sessionId;
+    }
+    if (this._mainSessionId === sessionId) {
+      sessionId = undefined;
+    }
+
     let request = this._recordsById.get(requestId);
     if (!request || !request.isValid) return undefined;