[DevTools] Make DevToolsSession own it's DevToolsMessageChunkProcessor

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 774c925a8d9f0fa5ab56a26d7a197fb52ac73548..6b2a7851949819e230bdcce6f83aa212c29478dd 100644
--- a/content/browser/devtools/devtools_agent_host_impl.h
+++ b/content/browser/devtools/devtools_agent_host_impl.h
@@ -91,7 +91,7 @@ class DevToolsMessageChunkProcessor {
   explicit DevToolsMessageChunkProcessor(const SendMessageCallback& callback);
   ~DevToolsMessageChunkProcessor();
 
-  std::string state_cookie() const { return state_cookie_; }
+  const std::string& state_cookie() const { return state_cookie_; }
   void set_state_cookie(const std::string& cookie) { state_cookie_ = cookie; }
   int last_call_id() const { return last_call_id_; }
   bool ProcessChunkedMessageFromAgent(const DevToolsMessageChunk& chunk);