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

content/shell/shell_render_view_observer.h

Index: content/shell/shell_render_view_observer.h
diff --git a/content/shell/shell_render_view_observer.h b/content/shell/shell_render_view_observer.h
index c1d963fbdd6c2486441cde8d1dce64b31b1a6503..ad5466bd189cf072f8082fa7e7b0866ab2cb75f6 100644
--- a/content/shell/shell_render_view_observer.h
+++ b/content/shell/shell_render_view_observer.h
@@ -21,6 +21,9 @@ class ShellRenderViewObserver : public RenderViewObserver {
   virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;
 
  private:
+  // Message handlers.
+  void OnCaptureTextDump(bool recursive);
+
   DISALLOW_COPY_AND_ASSIGN(ShellRenderViewObserver);
 };