Plumb Blink notification actions to button UI on desktop

content/common/notification_constants.h

Index: content/common/notification_constants.h
diff --git a/content/common/notification_constants.h b/content/common/notification_constants.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 CONTENT_COMMON_NOTIFICATION_CONSTANTS_H_
+#define CONTENT_COMMON_NOTIFICATION_CONSTANTS_H_
+
+namespace content {
+
+// Maximum number of actions on a Platform Notification.
+static const size_t kPlatformNotificationMaxActions = 2;
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_NOTIFICATION_CONSTANTS_H_