Fix crashing test flake

Found this flake in an in-flight CL so I am confident this will fix
the issue without disabling.

Bug: 1060454
Change-Id: Ie5420a260c90e94a825428099483e0258996c370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111069
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751852}

chrome/browser/prerender/isolated/isolated_prerender_browsertest.cc

@@ -86,7 +86,9 @@ class TestCustomProxyConfigClient
   void OnCustomProxyConfigUpdated(
       network::mojom::CustomProxyConfigPtr proxy_config) override {
     config_ = std::move(proxy_config);
-    std::move(update_closure_).Run();
+    if (update_closure_) {
+      std::move(update_closure_).Run();
+    }
   }
   void MarkProxiesAsBad(base::TimeDelta bypass_duration,
                         const net::ProxyList& bad_proxies,