Really fix unused-variable warnings

I misunderstood how ALLOW_UNUSED is meant to be used. Using a macro like
MSV_WARNING_SUPPRESS() would be nice, but for some reason it isn't working for
me. So now we just manually reference the variable in the code.

brightray/browser/url_request_context_getter.cc

@@ -154,8 +154,8 @@ net::URLRequestContext* URLRequestContextGetter::GetURLRequestContext() {
         end = protocol_handlers_.end(); it != end; ++it) {
       bool set_protocol = job_factory->SetProtocolHandler(
           it->first, it->second.release());
-      ALLOW_UNUSED(set_protocol);
       DCHECK(set_protocol);
+      (void)set_protocol; // silence unused-variable warning in Release builds on Windows
     }
     protocol_handlers_.clear();
     job_factory->SetProtocolHandler(