[headless] Make browser service manifest overlay configurable.

headless/public/headless_browser.cc

Index: headless/public/headless_browser.cc
diff --git a/headless/public/headless_browser.cc b/headless/public/headless_browser.cc
index cbfbc5e400e682bc6223b5838b2bc252a94a305c..2b4270bd591f333ee69a287b007c1e0030e841e7 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -82,6 +82,11 @@ Builder& Builder::SetGLImplementation(const std::string& gl_implementation) {
   return *this;
 }
 
+Builder& Builder::AddMojoServiceName(const std::string& mojo_service_name) {
+  options_.mojo_service_names.insert(mojo_service_name);
+  return *this;
+}
+
 Builder& Builder::SetUserDataDir(const base::FilePath& user_data_dir) {
   options_.user_data_dir = user_data_dir;
   return *this;