Remove task.h and finish base::Bind() migration.

base/bind_helpers.cc

Index: base/bind_helpers.cc
diff --git a/base/task.cc b/base/bind_helpers.cc
similarity index 86%
rename from base/task.cc
rename to base/bind_helpers.cc
index baf4e0f6bb1f28622f78961ed8ee70c88286e4ce..f2fc3bb0daaed9697a899b2e80f5322e1a074fe9 100644
--- a/base/task.cc
+++ b/base/bind_helpers.cc
@@ -2,10 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/task.h"
+#include "base/bind_helpers.h"
+
+#include "base/callback.h"
 
 namespace base {
 
+void DoNothing() {
+}
+
 ScopedClosureRunner::ScopedClosureRunner(const Closure& closure)
     : closure_(closure) {
 }