webauthn: don't crash when scanning a second QR code.

Nulling out the permissions delegate would cause a crash if the user
started scanning a QR code, abandoned, and then tried again.

BUG=1002262

Change-Id: I5cd3345d312dc80588e5e8a7df67d613fca83318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511972
Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#823348}

chrome/android/features/cablev2_authenticator/java/src/org/chromium/chrome/browser/webauth/authenticator/CableAuthenticatorUI.java

@@ -165,8 +165,6 @@ public class CableAuthenticatorUI extends Fragment
     @Override
     public void onRequestPermissionsResult(
             int requestCode, String[] permissions, int[] grantResults) {
-        mPermissionDelegate = null;
-
         if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
             (new QRScanDialog(this)).show(getFragmentManager(), "dialog");
         }