headless: Enable GL support 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 b1e5dbcc026ec701c2dab1ba3e5f6be04e990d45..3905ec0b4dc2694ff3b00009aed39d56716395a4 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -20,7 +20,8 @@ Options::Options(int argc, const char** argv)
       argv(argv),
       user_agent(content::BuildUserAgentFromProduct(kProductName)),
       message_pump(nullptr),
-      single_process_mode(false) {}
+      single_process_mode(false),
+      gl_implementation("osmesa") {}
 
 Options::Options(Options&& options) = default;
 
@@ -69,6 +70,11 @@ Builder& Builder::SetProtocolHandlers(ProtocolHandlerMap protocol_handlers) {
   return *this;
 }
 
+Builder& Builder::SetGLImplementation(const std::string& gl_implementation) {
+  options_.gl_implementation = gl_implementation;
+  return *this;
+}
+
 Options Builder::Build() {
   return std::move(options_);
 }