Fix WebAuthFlow crash of dereferencing nullptr navigation_handle->GetResponseHeaders().

chrome/browser/extensions/api/identity/web_auth_flow.cc

Index: chrome/browser/extensions/api/identity/web_auth_flow.cc
diff --git a/chrome/browser/extensions/api/identity/web_auth_flow.cc b/chrome/browser/extensions/api/identity/web_auth_flow.cc
index 663f17437b53157892c60bc96debf8a5eb6e0924..c6c84168512b9e6464435389028b7075638c6c1a 100644
--- a/chrome/browser/extensions/api/identity/web_auth_flow.cc
+++ b/chrome/browser/extensions/api/identity/web_auth_flow.cc
@@ -212,6 +212,7 @@ void WebAuthFlow::DidFinishNavigation(
                                  "DidFinishNavigationFailure", "error_code",
                                  navigation_handle->GetNetErrorCode());
   } else if (navigation_handle->IsInMainFrame() &&
+             navigation_handle->GetResponseHeaders() &&
              navigation_handle->GetResponseHeaders()->response_code() >= 400) {
     failed = true;
     TRACE_EVENT_ASYNC_STEP_PAST1(