headless: Add devtools discovery page

headless/lib/browser/headless_devtools.cc

Index: headless/lib/browser/headless_devtools.cc
diff --git a/headless/lib/browser/headless_devtools.cc b/headless/lib/browser/headless_devtools.cc
index 8fe4b7d10b986d55b78600f8ede0930e7c6617d3..258e9de2f96e1982b5d672b7e9077dff0e007e9c 100644
--- a/headless/lib/browser/headless_devtools.cc
+++ b/headless/lib/browser/headless_devtools.cc
@@ -10,6 +10,7 @@
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/devtools_frontend_host.h"
 #include "content/public/browser/navigation_entry.h"
+#include "headless/grit/headless_lib_resources.h"
 #include "headless/lib/browser/headless_browser_context.h"
 #include "net/base/net_errors.h"
 #include "net/socket/tcp_server_socket.h"
@@ -68,7 +69,8 @@ HeadlessDevToolsDelegate::HeadlessDevToolsDelegate() {}
 HeadlessDevToolsDelegate::~HeadlessDevToolsDelegate() {}
 
 std::string HeadlessDevToolsDelegate::GetDiscoveryPageHTML() {
-  return std::string();
+  return ResourceBundle::GetSharedInstance().GetRawDataResource(
+      IDR_HEADLESS_LIB_DEVTOOLS_DISCOVERY_PAGE).as_string();
 }
 
 std::string HeadlessDevToolsDelegate::GetFrontendResource(