Update to Chromium revision 12bfc336 (#338390).

- The ffmpeg library is now statically linked (see https://codereview.chromium.org/1141703002).
- Off-screen rendering of the PDF viewer does not work in combination with surfaces. Pass the
  `--disable-surfaces` command-line flag if GPU is enabled (see https://codereview.chromium.org/1169983006).

libcef/browser/url_request_context_getter_impl.cc

@@ -184,6 +184,7 @@ net::URLRequestContext* CefURLRequestContextGetterImpl::GetURLRequestContext() {
             url_security_manager_.get(),
             url_request_context_->host_resolver(),
             std::string(),
+            std::string(),
             false,
             false));
     storage_->set_http_server_properties(
@@ -372,5 +373,5 @@ void CefURLRequestContextGetterImpl::CreateProxyConfigService() {
 
   proxy_config_service_.reset(
       net::ProxyService::CreateSystemProxyConfigService(
-          io_loop_->message_loop_proxy(), file_loop_->message_loop_proxy()));
+          io_loop_->task_runner(), file_loop_->task_runner()));
 }