Make ChromeAuthenticatorDialogDelegate clean up more tidy.

This is a follow-up to crrev.com/c/964448, which was committed before
a comment around this was addressed.

Bug: 849323
Change-Id: I5c6de00476424a566081a55b5bb2f2ef787c4cb6
TBR: tapted@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1092857
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565636}

chrome/browser/webauthn/chrome_authenticator_request_delegate.cc

@@ -73,9 +73,13 @@ ChromeAuthenticatorRequestDelegate::~ChromeAuthenticatorRequestDelegate() {
   // Currently, completion of the request is indicated by //content destroying
   // this delegate.
   if (weak_dialog_model_) {
+    weak_dialog_model_->OnRequestComplete();
+  }
+
   // The dialog model may be destroyed after the OnRequestComplete call.
+  if (weak_dialog_model_) {
     weak_dialog_model_->RemoveObserver(this);
-    weak_dialog_model_->OnRequestComplete();
+    weak_dialog_model_ = nullptr;
   }
 }