Null-check WindowAndroid pointer in chrome_signin_helper.

Bug: 1084214
Change-Id: I0a10324c305438d01873cf1415c35ffc94e597a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207757
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770120}

chrome/browser/signin/chrome_signin_helper.cc

@@ -304,6 +304,8 @@ void ProcessMirrorHeader(
     signin_metrics::LogAccountReconcilorStateOnGaiaResponse(
         account_reconcilor->GetState());
     auto* window = web_contents->GetNativeView()->GetWindowAndroid();
+    if (!window)
+      return;
     SigninUtils::OpenAccountManagementScreen(window, service_type,
                                              manage_accounts_params.email);
   }