[DevTools] Use InspectorFrontendHost.readyForTest for layout tests.

content/shell/browser/layout_test/layout_test_devtools_frontend.h

Index: content/shell/browser/layout_test/layout_test_devtools_frontend.h
diff --git a/content/shell/browser/layout_test/layout_test_devtools_frontend.h b/content/shell/browser/layout_test/layout_test_devtools_frontend.h
index db2dc4168dc9a00d9d9d8bff924748ff258147ec..839a9a56938c349713a5ae5bf8adbfdf85b71291 100644
--- a/content/shell/browser/layout_test/layout_test_devtools_frontend.h
+++ b/content/shell/browser/layout_test/layout_test_devtools_frontend.h
@@ -26,6 +26,7 @@ class LayoutTestDevToolsFrontend : public ShellDevToolsFrontend {
 
   void ReuseFrontend(const std::string& settings,
                      const std::string frontend_url);
+  void EvaluateInFrontend(int call_id, const std::string& expression);
 
  private:
   LayoutTestDevToolsFrontend(Shell* frontend_shell,
@@ -35,9 +36,15 @@ class LayoutTestDevToolsFrontend : public ShellDevToolsFrontend {
   // content::DevToolsAgentHostClient implementation.
   void AgentHostClosed(DevToolsAgentHost* agent_host, bool replaced) override;
 
+  // ShellDevToolsFrontend overrides.
+  void HandleMessageFromDevToolsFrontend(const std::string& message) override;
+
   // WebContentsObserver implementation.
   void RenderProcessGone(base::TerminationStatus status) override;
 
+  bool ready_for_test_;
+  std::vector<std::pair<int, std::string>> pending_evaluations_;
+
   DISALLOW_COPY_AND_ASSIGN(LayoutTestDevToolsFrontend);
 };