Add missing (and remove superfluous) 'explicit' from constructors.

jingle/notifier/listener/non_blocking_push_client.cc

Index: jingle/notifier/listener/non_blocking_push_client.cc
diff --git a/jingle/notifier/listener/non_blocking_push_client.cc b/jingle/notifier/listener/non_blocking_push_client.cc
index c42561e685a7dedd4fbe9741d66c9cf04c023f01..aaf0f639920835ea07b7077bd4c471c7dc12a33c 100644
--- a/jingle/notifier/listener/non_blocking_push_client.cc
+++ b/jingle/notifier/listener/non_blocking_push_client.cc
@@ -19,9 +19,8 @@ class NonBlockingPushClient::Core
       public PushClientObserver {
  public:
   // Called on the parent thread.
-  explicit Core(
-      const scoped_refptr<base::SingleThreadTaskRunner>&
-          delegate_task_runner,
+  Core(
+      const scoped_refptr<base::SingleThreadTaskRunner>& delegate_task_runner,
       const base::WeakPtr<NonBlockingPushClient>& parent_push_client);
 
   // Must be called after being created.