[PresentationAPI] Implementing send() from WebPresentationClient to the PresentationService.

content/public/common/presentation_constants.h

+// 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_PUBLIC_COMMON_PRESENTATION_CONSTANTS_H_
+#define CONTENT_PUBLIC_COMMON_PRESENTATION_CONSTANTS_H_
+
+#include <stddef.h>         // For size_t
+
+#include "content/common/content_export.h"
+
+namespace content {
+
+// The maximum number of bytes allowed in a presentation session message.
+CONTENT_EXPORT extern const size_t kMaxPresentationSessionMessageSize;
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_COMMON_PRESENTATION_CONSTANTS_H_