[headless] Make browser service manifest overlay configurable.

headless/lib/browser/headless_content_browser_client.cc

Index: headless/lib/browser/headless_content_browser_client.cc
diff --git a/headless/lib/browser/headless_content_browser_client.cc b/headless/lib/browser/headless_content_browser_client.cc
index b863c60fe58e942385a6e0e3b8eb2b52bc84324d..53a4688106e176a8c97e210af4d93f3b9f59cc83 100644
--- a/headless/lib/browser/headless_content_browser_client.cc
+++ b/headless/lib/browser/headless_content_browser_client.cc
@@ -56,11 +56,13 @@ HeadlessContentBrowserClient::GetDevToolsManagerDelegate() {
 std::unique_ptr<base::Value>
 HeadlessContentBrowserClient::GetServiceManifestOverlay(
     const std::string& name) {
-  if (name != content::kBrowserServiceName)
+  if (name != content::kBrowserServiceName ||
+      !browser_->options()->mojo_service_manifest_resource_id)
     return nullptr;
+
   base::StringPiece manifest_contents =
       ui::ResourceBundle::GetSharedInstance().GetRawDataResource(
-          IDR_HEADLESS_BROWSER_MANIFEST_OVERLAY);
+          browser_->options()->mojo_service_manifest_resource_id);
   return base::JSONReader::Read(manifest_contents);
 }