clients(lr): increase Page.getAppManifest timeout to 10s (#8350)

lighthouse-core/gather/driver.js

@@ -525,7 +525,14 @@ class Driver {
    * @return {Promise<{url: string, data: string}|null>}
    */
   async getAppManifest() {
-    this.setNextProtocolTimeout(3000);
+    // In all environments but LR, Page.getAppManifest finishes very quickly.
+    // In LR, there is a bug that causes this command to hang until outgoing
+    // requests finish. This has been seen in long polling (where it will never
+    // return) and when other requests take a long time to finish. We allow 10 seconds
+    // for outgoing requests to finish. Anything more, and we continue the run without
+    // a manifest.
+    // Googlers, see: http://b/124008171
+    this.setNextProtocolTimeout(10000);
     let response;
     try {
       response = await this.sendCommand('Page.getAppManifest');