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

content/public/browser/devtools_manager_delegate.cc

Index: content/public/browser/devtools_manager_delegate.cc
diff --git a/content/public/browser/devtools_manager_delegate.cc b/content/public/browser/devtools_manager_delegate.cc
index d3a9608fb09c090136833945821099de88a124c2..3e314535ecb4365871c5e4fd43722d0da8f0732d 100644
--- a/content/public/browser/devtools_manager_delegate.cc
+++ b/content/public/browser/devtools_manager_delegate.cc
@@ -15,7 +15,7 @@ void DevToolsManagerDelegate::DevToolsAgentStateChanged(
 }
 
 std::string DevToolsManagerDelegate::GetTargetType(RenderFrameHost* host) {
-  return DevToolsAgentHost::kTypePage;
+  return std::string();
 }
 
 std::string DevToolsManagerDelegate::GetTargetTitle(RenderFrameHost* host) {