PaymentApp: Add PaymentAppProvider class.

content/browser/payments/payment_app_provider_impl.h

+// 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 CONTENT_BROWSER_PAYMENTS_PAYMENT_APP_PROVIDER_IMPL_H_
+#define CONTENT_BROWSER_PAYMENTS_PAYMENT_APP_PROVIDER_IMPL_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "base/memory/singleton.h"
+#include "components/payments/payment_app.mojom.h"
+#include "content/common/content_export.h"
+#include "content/public/browser/payment_app_provider.h"
+
+namespace content {
+
+class BrowserContext;
+
+class CONTENT_EXPORT PaymentAppProviderImpl : public PaymentAppProvider {
+ public:
+  static PaymentAppProviderImpl* GetInstance();
+
+  // PaymentAppProvider implementation:
+  // Should be accessed only on the UI thread.

[please use gerrit instead, 2017/01/04 16:55:43]

"Should be accessed only on the UI thread." -- This comment should be in
PaymentAppProvider header file.

[zino, 2017/01/04 17:29:59]

I already left a comment at the top of the class definition.
But I will leave this comment on the method again.

Done.

+  void GetAllManifests(BrowserContext* browser_context,
+                       const GetAllManifestsCallback& callback) override;
+
+ private:
+  PaymentAppProviderImpl();
+  ~PaymentAppProviderImpl() override;
+
+  friend struct base::DefaultSingletonTraits<PaymentAppProviderImpl>;
+
+  DISALLOW_COPY_AND_ASSIGN(PaymentAppProviderImpl);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_PAYMENTS_PAYMENT_APP_PROVIDER_IMPL_H_