[FedCM] Prototype User Info API behind a flag (2/3)

User Info API can be used by IDPs to render personalized buttons. See
proposal at w3c-fedid/FedCM#382.

This patch:
 - Implement the proposal to return the requested user info.

Next:
 - Add metrics and console logs

Bug: 1304402
Change-Id: Ie9a970d4be4c4ca0e5d4cb3879823517b74fabcc

credential-management/support/fedcm-mock.js

@@ -80,6 +80,16 @@ export class MockFederatedAuthRequest {
     this.pendingPromiseResolve_ = null;
   }
 
+  // Implements
+  //   RequestUserInfo(IdentityProviderGetParameters idp_get_param) =>
+  //                    (RequestUserInfoStatus status, array<IdentityUserInfo>? user_info);
+  async requestUserInfo(idp_get_param) {
+    return Promise.resolve({
+      status: "",
+      user_info: ""
+    });
+  }
+
   async logoutRps(logout_endpoints) {
     return Promise.resolve({
       status: this.logoutRpsStatus_