headless: Make it possible to configure an HTTP proxy

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 550d7169c4cb87af19c017449a9679b7a4911fac..abadc18b9875f73e620c4914bf8df8bdabd7bb16 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -26,6 +26,8 @@ Options::~Options() {}
 
 Builder::Builder(int argc, const char** argv) : options_(argc, argv) {}
 
+Builder::Builder() : options_(0, nullptr) {}
+
 Builder::~Builder() {}
 
 Builder& Builder::SetUserAgent(const std::string& user_agent) {
@@ -43,6 +45,11 @@ Builder& Builder::SetMessagePump(base::MessagePump* message_pump) {
   return *this;
 }
 
+Builder& Builder::SetProxyServer(const net::HostPortPair& proxy_server) {
+  options_.proxy_server = proxy_server;
+  return *this;
+}
+
 Options Builder::Build() {
   return options_;
 }