DevTools: add custom-devtools-frontend flag for content shell layout 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 bd30d22ff34bbf9406ccb3696307d7659ffd6572..d12ddd0e2f21089c8ff4f3c9737db981d05ba866 100644
--- a/content/shell/browser/layout_test/layout_test_devtools_frontend.cc
+++ b/content/shell/browser/layout_test/layout_test_devtools_frontend.cc
@@ -52,13 +52,20 @@ GURL LayoutTestDevToolsFrontend::GetDevToolsPathAsURL(
   // We need to go up 3 directories to get to out/Release.
   dir_exe = dir_exe.AppendASCII("../../..");
 #endif
+  base::FilePath dev_tools_path;
   bool is_debug_dev_tools = base::CommandLine::ForCurrentProcess()->HasSwitch(
       switches::kDebugDevTools);
-  std::string folder = is_debug_dev_tools ? "debug/" : "";
-  base::FilePath dev_tools_path =
-      dir_exe.AppendASCII("resources/inspector/" + folder + "inspector.html");
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kCustomDevToolsFrontend)) {
+    dev_tools_path = base::CommandLine::ForCurrentProcess()->GetSwitchValuePath(
+        switches::kCustomDevToolsFrontend);
+  } else {
+    std::string folder = is_debug_dev_tools ? "debug/" : "";
+    dev_tools_path = dir_exe.AppendASCII("resources/inspector/" + folder);
+  }
 
-  GURL result = net::FilePathToFileURL(dev_tools_path);
+  GURL result =
+      net::FilePathToFileURL(dev_tools_path.AppendASCII("inspector.html"));
   std::string url_string =
       base::StringPrintf("%s?experiments=true", result.spec().c_str());
   if (is_debug_dev_tools)