Add the null pointer checking to resolve the crash issue when shutdown.

Click the open with button at the browser url bar, then shutdown the
system, it crashes due to access the null pointer. So add the null
pointer checking to avoid the system crash.

BUG=1116344

Change-Id: I4c45983f8d626eabd98a8e0f188c32255ffbe0b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353779
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798281}

chrome/browser/ui/ash/launcher/app_service/app_service_instance_registry_helper.cc

@@ -331,8 +331,12 @@ void AppServiceInstanceRegistryHelper::SetWindowActivated(
     Browser* browser = chrome::FindBrowserWithWindow(window);
     if (!browser)
       return;
+
     content::WebContents* contents =
         browser->tab_strip_model()->GetActiveWebContents();
+    if (!contents)
+      return;
+
     apps::InstanceState state = static_cast<apps::InstanceState>(
         apps::InstanceState::kStarted | apps::InstanceState::kRunning |
         apps::InstanceState::kActive | apps::InstanceState::kVisible);