Create content::WebContentsDestroyedWatcher, use it in many tests.

chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc

Index: chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc
diff --git a/chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc b/chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc
index 3addbf6dd80bd8453f13d2d8c68a3b2867dffff7..830663bce328e9d713a7852b12efe38e4b83429f 100644
--- a/chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc
+++ b/chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc
@@ -49,6 +49,7 @@
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_source.h"
 #include "content/public/browser/web_contents.h"
+#include "content/public/test/browser_test_utils.h"
 #include "extensions/common/switches.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/app_list/views/apps_grid_view.h"
@@ -846,11 +847,9 @@ IN_PROC_BROWSER_TEST_F(LauncherAppBrowserTest, LaunchPinned) {
   EXPECT_EQ(++tab_count, tab_strip->count());
   EXPECT_EQ(ash::STATUS_ACTIVE, (*model_->ItemByID(shortcut_id)).status);
   WebContents* tab = tab_strip->GetActiveWebContents();
-  content::WindowedNotificationObserver close_observer(
-      content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-      content::Source<WebContents>(tab));
+  content::WebContentsDestroyedWatcher destroyed_watcher(tab);
   browser()->tab_strip_model()->CloseSelectedTabs();
-  close_observer.Wait();
+  destroyed_watcher.Wait();
   EXPECT_EQ(--tab_count, tab_strip->count());
   EXPECT_EQ(ash::STATUS_CLOSED, (*model_->ItemByID(shortcut_id)).status);
 }
@@ -865,11 +864,9 @@ IN_PROC_BROWSER_TEST_F(LauncherAppBrowserTest, LaunchUnpinned) {
   ash::LauncherID shortcut_id = CreateShortcut("app1");
   EXPECT_EQ(ash::STATUS_ACTIVE, (*model_->ItemByID(shortcut_id)).status);
   WebContents* tab = tab_strip->GetActiveWebContents();
-  content::WindowedNotificationObserver close_observer(
-      content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-      content::Source<WebContents>(tab));
+  content::WebContentsDestroyedWatcher destroyed_watcher(tab);
   browser()->tab_strip_model()->CloseSelectedTabs();
-  close_observer.Wait();
+  destroyed_watcher.Wait();
   EXPECT_EQ(--tab_count, tab_strip->count());
   EXPECT_EQ(ash::STATUS_CLOSED, (*model_->ItemByID(shortcut_id)).status);
 }