[DevTools] Fix a couple of issues for cross-site RFDTAH, add a test.

content/public/browser/devtools_manager_delegate.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/devtools_manager_delegate.h"
 
 namespace content {
 
 void DevToolsManagerDelegate::Inspect(DevToolsAgentHost* agent_host) {
 }
 
 void DevToolsManagerDelegate::DevToolsAgentStateChanged(
     DevToolsAgentHost* agent_host, bool attached) {
 }
 
 std::string DevToolsManagerDelegate::GetTargetType(RenderFrameHost* host) {
-  return DevToolsAgentHost::kTypePage;
+  return std::string();
 }
 
 std::string DevToolsManagerDelegate::GetTargetTitle(RenderFrameHost* host) {
   return std::string();
 }
 
 std::string DevToolsManagerDelegate::GetTargetDescription(
     RenderFrameHost* host) {
   return std::string();
 }
@@ skipping 20 matching lines @@
 
 std::string DevToolsManagerDelegate::GetFrontendResource(
     const std::string& path) {
   return std::string();
 }
 
 DevToolsManagerDelegate::~DevToolsManagerDelegate() {
 }
 
 }  // namespace content