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

content/shell/shell_render_view_host_observer.h

Index: content/shell/shell_render_view_host_observer.h
diff --git a/content/shell/shell_render_view_host_observer.h b/content/shell/shell_render_view_host_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..f1ba4498e7a00a578b9da987893ac8eacb31ba55
--- /dev/null
+++ b/content/shell/shell_render_view_host_observer.h
@@ -0,0 +1,32 @@
+// 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.
+
+#ifndef CONTENT_SHELL_SHELL_RENDER_VIEW_HOST_OBSERVER_H_
+#define CONTENT_SHELL_SHELL_RENDER_VIEW_HOST_OBSERVER_H_
+#pragma once
+
+#include <string>
+
+#include "content/public/browser/render_view_host_observer.h"
+
+namespace content {
+
+class ShellRenderViewHostObserver : public RenderViewHostObserver {
+ public:
+  explicit ShellRenderViewHostObserver(RenderViewHost* render_view_host);
+  virtual ~ShellRenderViewHostObserver();
+
+  // RenderViewHostObserver implementation.
+  virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;
+
+ private:
+  // Message handlers.
+  void OnTextDump(const std::string& dump);
+
+  DISALLOW_COPY_AND_ASSIGN(ShellRenderViewHostObserver);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_SHELL_SHELL_RENDER_VIEW_HOST_OBSERVER_H_