Revert of Deletes mojo::Callback

mojo/public/cpp/bindings/tests/request_response_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdint.h>
 #include <utility>
 
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "mojo/public/cpp/test_support/test_utils.h"
 #include "mojo/public/interfaces/bindings/tests/sample_import.mojom.h"
 #include "mojo/public/interfaces/bindings/tests/sample_interfaces.mojom.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace mojo {
 namespace test {
 namespace {
 
 class ProviderImpl : public sample::Provider {
  public:
   explicit ProviderImpl(InterfaceRequest<sample::Provider> request)
       : binding_(this, std::move(request)) {}
 
   void EchoString(const String& a,
-                  const EchoStringCallback& callback) override {
-    EchoStringCallback callback_copy;
+                  const Callback<void(String)>& callback) override {
+    Callback<void(String)> callback_copy;
     // Make sure operator= is used.
     callback_copy = callback;
     callback_copy.Run(a);
   }
 
   void EchoStrings(const String& a,
                    const String& b,
-                   const EchoStringsCallback& callback) override {
+                   const Callback<void(String, String)>& callback) override {
     callback.Run(a, b);
   }
 
   void EchoMessagePipeHandle(
       ScopedMessagePipeHandle a,
-      const EchoMessagePipeHandleCallback& callback) override {
+      const Callback<void(ScopedMessagePipeHandle)>& callback) override {
     callback.Run(std::move(a));
   }
 
-  void EchoEnum(sample::Enum a, const EchoEnumCallback& callback) override {
+  void EchoEnum(sample::Enum a,
+                const Callback<void(sample::Enum)>& callback) override {
     callback.Run(a);
   }
 
   void EchoInt(int32_t a, const EchoIntCallback& callback) override {
     callback.Run(a);
   }
 
   Binding<sample::Provider> binding_;
 };
 
@@ skipping 93 matching lines @@
   provider->EchoEnum(sample::Enum::VALUE,
                      base::Bind(&RecordEnum, &value, run_loop.QuitClosure()));
   run_loop.Run();
 
   EXPECT_EQ(sample::Enum::VALUE, value);
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace mojo