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

chrome/browser/browser_keyevents_browsertest.cc

Index: chrome/browser/browser_keyevents_browsertest.cc
diff --git a/chrome/browser/browser_keyevents_browsertest.cc b/chrome/browser/browser_keyevents_browsertest.cc
index 77f10c47831fc46b266f2e573be723051e57f2ee..0a2580839fe3d62b6308daff86fcceb9ccb44fed 100644
--- a/chrome/browser/browser_keyevents_browsertest.cc
+++ b/chrome/browser/browser_keyevents_browsertest.cc
@@ -723,10 +723,8 @@ IN_PROC_BROWSER_TEST_F(BrowserKeyEventsTest, MAYBE_ReservedAccelerators) {
   // Reserved accelerators can't be suppressed.
   ASSERT_NO_FATAL_FAILURE(SuppressAllEvents(1, true));
 
-  content::WindowedNotificationObserver wait_for_tab_closed(
-      content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-      content::Source<content::WebContents>(
-          browser()->tab_strip_model()->GetWebContentsAt(1)));
+  content::WebContentsDestroyedWatcher destroyed_watcher(
+      browser()->tab_strip_model()->GetWebContentsAt(1));
 
   // Press Ctrl/Cmd+W, which will close the tab.
 #if defined(OS_MACOSX)
@@ -737,7 +735,7 @@ IN_PROC_BROWSER_TEST_F(BrowserKeyEventsTest, MAYBE_ReservedAccelerators) {
       browser(), ui::VKEY_W, true, false, false, false));
 #endif
 
-  ASSERT_NO_FATAL_FAILURE(wait_for_tab_closed.Wait());
+  ASSERT_NO_FATAL_FAILURE(destroyed_watcher.Wait());
 
   EXPECT_EQ(1, browser()->tab_strip_model()->count());
 }