[DevTools] Use InspectorFrontendHost.readyForTest for layout tests.

content/shell/browser/shell_devtools_frontend.h

Index: content/shell/browser/shell_devtools_frontend.h
diff --git a/content/shell/browser/shell_devtools_frontend.h b/content/shell/browser/shell_devtools_frontend.h
index b1be4a646b48d6c841fda69b26eb83522ce27fa9..3258a5ac53851fbb986ef5ffacbdd61423a7b55c 100644
--- a/content/shell/browser/shell_devtools_frontend.h
+++ b/content/shell/browser/shell_devtools_frontend.h
@@ -55,6 +55,7 @@ class ShellDevToolsFrontend : public WebContentsObserver,
   void DispatchProtocolMessage(DevToolsAgentHost* agent_host,
                                const std::string& message) override;
   base::DictionaryValue* preferences() { return &preferences_; }
+  virtual void HandleMessageFromDevToolsFrontend(const std::string& message);
 
  private:
   // WebContentsObserver overrides
@@ -62,8 +63,6 @@ class ShellDevToolsFrontend : public WebContentsObserver,
   void DocumentAvailableInMainFrame() override;
   void WebContentsDestroyed() override;
 
-  void HandleMessageFromDevToolsFrontend(const std::string& message);
-
   // net::URLFetcherDelegate overrides.
   void OnURLFetchComplete(const net::URLFetcher* source) override;