[DevTools] use debug_frontend flag in content_shell for tests

content/shell/browser/layout_test/layout_test_devtools_frontend.cc

Index: content/shell/browser/layout_test/layout_test_devtools_frontend.cc
diff --git a/content/shell/browser/layout_test/layout_test_devtools_frontend.cc b/content/shell/browser/layout_test/layout_test_devtools_frontend.cc
index 63a7809f015c0a93372c9a99a8e6899546319532..8fb72565bbc65d95d874a29562343ee8969e00e6 100644
--- a/content/shell/browser/layout_test/layout_test_devtools_frontend.cc
+++ b/content/shell/browser/layout_test/layout_test_devtools_frontend.cc
@@ -53,11 +53,14 @@ GURL LayoutTestDevToolsFrontend::GetDevToolsPathAsURL(
       dir_exe.AppendASCII("resources/inspector/inspector.html");
 
   GURL result = net::FilePathToFileURL(dev_tools_path);
+  std::string url_string =
+      base::StringPrintf("%s?experiments=true", result.spec().c_str());
+#if defined(DEBUG_DEVTOOLS)
+  url_string += "&debugFrontend=true";
+#endif  // defined(DEBUG_DEVTOOLS)
   if (!settings.empty())
-    result = GURL(base::StringPrintf("%s?settings=%s&experiments=true",
-                                     result.spec().c_str(),
-                                     settings.c_str()));
-  return result;
+    url_string += "&settings=" + settings;
+  return GURL(url_string);
 }
 
 void LayoutTestDevToolsFrontend::ReuseFrontend(const std::string& settings,