Create a RenderProcess message class and move keygen to it.

content/common/render_process_messages.h

Index: content/common/render_process_messages.h
diff --git a/content/common/render_process_messages.h b/content/common/render_process_messages.h
new file mode 100644
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+++ b/content/common/render_process_messages.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.
+
+// Common IPC messages used for render processes.
+// Multiply-included message file, hence no include guard.
+
+#include <string>
+
+#include "ipc/ipc_message_macros.h"
+#include "ipc/ipc_message_utils.h"
+#include "url/gurl.h"
+
+#undef IPC_MESSAGE_EXPORT
+#define IPC_MESSAGE_EXPORT CONTENT_EXPORT
+
+#define IPC_MESSAGE_START RenderProcessMsgStart
+
+////////////////////////////////////////////////////////////////////////////////
+// Messages sent from the browser to the render process.
+
+////////////////////////////////////////////////////////////////////////////////
+// Messages sent from the render process to the browser.
+
+// Asks the browser process to generate a keypair for grabbing a client
+// certificate from a CA (<keygen> tag), and returns the signed public
+// key and challenge string.
+IPC_SYNC_MESSAGE_CONTROL3_1(RenderProcessHostMsg_Keygen,
+                            uint32 /* key size index */,
+                            std::string /* challenge string */,
+                            GURL /* URL of requestor */,
+                            std::string /* signed public key and challenge */)