Introduce TabRestoreServiceClient and //chrome implementation.

components/sessions/core/tab_restore_service_client.h

Index: components/sessions/core/tab_restore_service_client.h
diff --git a/components/sessions/core/tab_restore_service_client.h b/components/sessions/core/tab_restore_service_client.h
new file mode 100644
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+++ b/components/sessions/core/tab_restore_service_client.h
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+// Copyright 2015 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 COMPONENTS_SESSIONS_CORE_TAB_RESTORE_SERVICE_CLIENT_H_
+#define COMPONENTS_SESSIONS_CORE_TAB_RESTORE_SERVICE_CLIENT_H_
+
+#include "base/callback.h"
+#include "base/memory/scoped_vector.h"
+#include "components/sessions/session_id.h"
+
+namespace base {
+class CancelableTaskTracker;
+}
+
+namespace sessions {
+struct SessionWindow;
+}
+
+namespace sessions {
+
+struct SessionWindow;
+
+// Callback from TabRestoreServiceClient::GetLastSession.
+// The second parameter is the id of the window that was last active.
+typedef base::Callback<void(ScopedVector<SessionWindow>, SessionID::id_type)>
+    GetLastSessionCallback;
+
+// A client interface that needs to be supplied to the tab restore service by
+// the embedder.
+class TabRestoreServiceClient {
+ public:
+  virtual ~TabRestoreServiceClient() {}
+
+  // Returns whether there is a previous session to load.
+  virtual bool HasLastSession() = 0;
+
+  // Fetches the contents of the last session, notifying the callback when
+  // done. If the callback is supplied an empty vector of SessionWindows
+  // it means the session could not be restored.
+  virtual void GetLastSession(const GetLastSessionCallback& callback,
+                              base::CancelableTaskTracker* tracker) = 0;
+};
+
+}  // namespace sessions
+
+#endif  // COMPONENTS_SESSIONS_CORE_TAB_RESTORE_SERVICE_CLIENT_H_