Add option --dump-render-tree to content_shell to dump the render tree as text.

content/shell/shell_messages.h

Index: content/shell/shell_messages.h
diff --git a/content/shell/shell_messages.h b/content/shell/shell_messages.h
new file mode 100644
index 0000000000000000000000000000000000000000..5e3c74165b97d7963403a8f60147a7a79b8e8916
--- /dev/null
+++ b/content/shell/shell_messages.h
@@ -0,0 +1,19 @@
+// Copyright (c) 2012 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.
+
+// Multiply-included file, no traditional include guard.
+#include <string>
+
+#include "ipc/ipc_message_macros.h"
+
+#define IPC_MESSAGE_START ShellMsgStart
+
+// Tells the render view to capture a text dump of the page. The render view
+// responds with a ShellViewHostMsg_TextDump.
+IPC_MESSAGE_ROUTED1(ShellViewMsg_CaptureTextDump,
+                    bool /* recursive */)
+
+// Send a text dump of the tab contents to the render host.
+IPC_MESSAGE_ROUTED1(ShellViewHostMsg_TextDump,
+                    std::string /* dump */)