headless: Allow protocol handler customization

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 07d263641ac90fdd82feffc9131a158c89593088..0b4e17f1e7e14143fadfae104a915ae9225ef286 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -65,6 +65,11 @@ Builder& Builder::SetSingleProcessMode(bool single_process_mode) {
   return *this;
 }
 
+Builder& Builder::SetProtocolHandlers(ProtocolHandlerMap protocol_handlers) {
+  options_.protocol_handlers = std::move(protocol_handlers);
+  return *this;
+}
+
 Options Builder::Build() {
   return std::move(options_);
 }