Introduce Identity Service and its initial usage

services/identity/identity_manager.h

Index: services/identity/identity_manager.h
diff --git a/services/identity/identity_manager.h b/services/identity/identity_manager.h
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+// Copyright 2017 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.
+
+#ifndef SERVICES_IDENTITY_IDENTITY_MANAGER_H_
+#define SERVICES_IDENTITY_IDENTITY_MANAGER_H_
+
+#include "services/identity/public/interfaces/identity_manager.mojom.h"
+
+class SigninManagerBase;
+
+namespace identity {
+
+class IdentityManager : public mojom::IdentityManager {
+ public:
+  static void Create(mojom::IdentityManagerRequest request,
+                     SigninManagerBase* signin_manager);
+
+  IdentityManager(SigninManagerBase* signin_manager);
+  ~IdentityManager() override;
+
+ private:
+  // mojom::IdentityManager:
+  void GetPrimaryAccountId(
+      const GetPrimaryAccountIdCallback& callback) override;
+
+  SigninManagerBase* signin_manager_;
+};
+
+}  // namespace identity
+
+#endif  // SERVICES_IDENTITY_IDENTITY_MANAGER_H_