Revert of Deletes mojo::Callback

mojo/public/cpp/bindings/BUILD.gn

 # 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.
 
 source_set("bindings") {
   sources = [
     "array.h",
     "array_traits.h",
     "array_traits_carray.h",
     "array_traits_standard.h",
@@ skipping 99 matching lines @@
     "string_traits_stl.h",
     "string_traits_string16.h",
     "string_traits_string_piece.h",
     "strong_binding.h",
     "struct_ptr.h",
     "sync_handle_registry.h",
     "type_converter.h",
   ]
 
   public_deps = [
+    ":callback",
     ":struct_traits",
     "//base",
     "//ipc:param_traits",
     "//mojo/public/cpp/system",
   ]
 
   deps = [
     "//base",
     "//mojo/public/interfaces/bindings:bindings_cpp_sources",
   ]
 }
 
+source_set("callback") {
+  sources = [
+    "callback.h",
+  ]
+
+  public_deps = [
+    "//base",
+  ]
+}
+
 source_set("struct_traits") {
   sources = [
     "struct_traits.h",
   ]
 }
 
 if (!is_ios) {
   # TODO(yzshen): crbug.com/617718 Consider moving this into blink.
   source_set("wtf_support") {
     sources = [
       "array_traits_wtf.h",
       "array_traits_wtf_vector.h",
       "lib/string_traits_wtf.cc",
       "lib/wtf_serialization.h",
       "map_traits_wtf.h",
       "string_traits_wtf.h",
       "wtf_array.h",
       "wtf_map.h",
     ]
 
     public_deps = [
       ":bindings",
       "//third_party/WebKit/Source/wtf",
     ]
 
     public_configs = [ "//third_party/WebKit/Source:config" ]
   }
 }