Simplify ExtensionInstallChecker into a single-use class

extensions/test/extension_test_notification_observer.h

Index: extensions/test/extension_test_notification_observer.h
diff --git a/extensions/test/extension_test_notification_observer.h b/extensions/test/extension_test_notification_observer.h
index 655084eb012e27f6f061643b054fffa4339794e3..418d567a65f5aeae82c66ec09ba5f2e9a3fb9634 100644
--- a/extensions/test/extension_test_notification_observer.h
+++ b/extensions/test/extension_test_notification_observer.h
@@ -46,7 +46,8 @@ class ExtensionTestNotificationObserver : public content::NotificationObserver {
   // crashed.
   bool WaitForExtensionCrash(const std::string& extension_id);
 
-  // Wait for the crx installer to be done. Returns true if it really is done.
+  // Wait for the crx installer to be done. Returns true if it has finished
+  // successfully.
   bool WaitForCrxInstallerDone();
 
   // Watch for the given event type from the given source.