headless: Switch to multiprocess mode by default

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 194eeac5212704cfbdb851b134348f954d10f527..6ff0a0c35ff00fa081f6429764dea3e1b469d2f0 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -20,7 +20,8 @@ Options::Options(int argc, const char** argv)
     : argc(argc),
       argv(argv),
       user_agent(content::BuildUserAgentFromProduct(kProductName)),
-      message_pump(nullptr) {}
+      message_pump(nullptr),
+      single_process_mode(false) {}
 
 Options::Options(const Options& other) = default;
 
@@ -57,6 +58,11 @@ Builder& Builder::SetHostResolverRules(const std::string& host_resolver_rules) {
   return *this;
 }
 
+Builder& Builder::SetSingleProcessMode(bool single_process_mode) {
+  options_.single_process_mode = single_process_mode;
+  return *this;
+}
+
 Options Builder::Build() {
   return options_;
 }