Revert of Deletes mojo::Callback

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

Index: mojo/public/cpp/bindings/tests/request_response_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/request_response_unittest.cc b/mojo/public/cpp/bindings/tests/request_response_unittest.cc
index 2007ad76e072f70a3c29d0e845bfb17f6e44b761..cb944f7209c3937db896c7e7a7d8ee73a051b0d9 100644
--- a/mojo/public/cpp/bindings/tests/request_response_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/request_response_unittest.cc
@@ -23,8 +23,8 @@
       : 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);
@@ -32,17 +32,18 @@
 
   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);
   }