Introduce DevToolsExternalAgentProxy interface

content/browser/devtools/devtools_agent_host_impl.h

Index: content/browser/devtools/devtools_agent_host_impl.h
diff --git a/content/browser/devtools/devtools_agent_host_impl.h b/content/browser/devtools/devtools_agent_host_impl.h
index 6e1f4812a71393b62af34d553596f44e37cb1d52..91f802a21dcd20e282f3ee7cdd9cc3df2a249573 100644
--- a/content/browser/devtools/devtools_agent_host_impl.h
+++ b/content/browser/devtools/devtools_agent_host_impl.h
@@ -27,11 +27,14 @@ class CONTENT_EXPORT DevToolsAgentHostImpl : public DevToolsAgentHost {
     virtual ~CloseListener() {}
   };
 
-  // Sends the message to the devtools agent hosted by this object.
-  void Attach();
-  void Reattach(const std::string& saved_agent_state);
-  void Detach();
-  virtual void DispatchOnInspectorBackend(const std::string& message);
+  // Informs the hosted agent that a client host has attached.
+  virtual void Attach() = 0;
+
+  // Informs the hosted agent that a client host has detached.
+  virtual void Detach() = 0;
+
+  // Sends a message to the agent hosted by this object.
+  virtual void DispatchOnInspectorBackend(const std::string& message) = 0;
 
   void set_close_listener(CloseListener* listener) {
     close_listener_ = listener;
@@ -48,10 +51,6 @@ class CONTENT_EXPORT DevToolsAgentHostImpl : public DevToolsAgentHost {
   DevToolsAgentHostImpl();
   virtual ~DevToolsAgentHostImpl();
 
-  virtual void SendMessageToAgent(IPC::Message* msg) = 0;
-  virtual void NotifyClientAttaching() = 0;
-  virtual void NotifyClientDetaching() = 0;
-
   void NotifyCloseListener();
 
  private: