Add WaitForCondition() and use it in ExtensionTestNotificationObserver.

base/callback_list.h.pump

Index: base/callback_list.h.pump
diff --git a/base/callback_list.h.pump b/base/callback_list.h.pump
index 168c70c2a3bf905e98ae0cbe3eec82e7565cd042..ea2103ebeefba2ad1567659c8d60deb99f8af9a6 100644
--- a/base/callback_list.h.pump
+++ b/base/callback_list.h.pump
@@ -110,7 +110,7 @@ class CallbackListBase {
   // Add a callback to the list. The callback will remain registered until the
   // returned Subscription is destroyed, which must occur before the
   // CallbackList is destroyed.
-  scoped_ptr<Subscription> Add(const CallbackType& cb) {
+  scoped_ptr<Subscription> Add(const CallbackType& cb) WARN_UNUSED_RESULT {
     DCHECK(!cb.is_null());
     return scoped_ptr<Subscription>(
         new Subscription(this, callbacks_.insert(callbacks_.end(), cb)));