cros: Fix crash after killing preloaded lock screen in task manager.

chrome/browser/chromeos/login/ui/preloaded_web_view.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/ui/preloaded_web_view.h"
 
 #include "base/callback_helpers.h"
 #include "base/memory/ptr_util.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/chromeos/idle_detector.h"
@@ skipping 25 matching lines @@
 void PreloadedWebView::PreloadOnIdle(PreloadCallback preload) {
   preload_function_ = std::move(preload);
   idle_detector_ = base::MakeUnique<chromeos::IdleDetector>(
       base::Bind(&PreloadedWebView::RunPreloader, weak_factory_.GetWeakPtr()));
   idle_detector_->Start(
       base::TimeDelta::FromSeconds(kIdleSecondsBeforePreloadingLockScreen));
 }
 
 std::unique_ptr<views::WebView> PreloadedWebView::TryTake() {
   idle_detector_.reset();
+
+  // Clear cached reference if it is no longer valid (ie, destroyed in task
+  // manager).
+  if (preloaded_instance_ &&
+      !preloaded_instance_->GetWebContents()
+           ->GetRenderViewHost()
+           ->GetWidget()
+           ->GetView()) {
+    preloaded_instance_.reset();
+  }
+
   return std::move(preloaded_instance_);
 }
 
 void PreloadedWebView::Shutdown() {
   preloaded_instance_.reset();
 }
 
 void PreloadedWebView::Observe(int type,
                                const content::NotificationSource& source,
                                const content::NotificationDetails& details) {
@@ skipping 12 matching lines @@
       break;
   }
 }
 
 void PreloadedWebView::RunPreloader() {
   idle_detector_.reset();
   preloaded_instance_ = std::move(preload_function_).Run(profile_);
 }
 
 }  // namespace chromeos