Add an accessor to HeadlessBrowser for the IO thread

headless/lib/browser/headless_browser_impl.cc

Index: headless/lib/browser/headless_browser_impl.cc
diff --git a/headless/lib/browser/headless_browser_impl.cc b/headless/lib/browser/headless_browser_impl.cc
index 57f32e529ba9b2b58aadc4fe46eec1c1093667a8..911fc6321122c43facc077acfbba04427f190567 100644
--- a/headless/lib/browser/headless_browser_impl.cc
+++ b/headless/lib/browser/headless_browser_impl.cc
@@ -63,15 +63,21 @@ HeadlessBrowserImpl::CreateBrowserContextBuilder() {
 }
 
 scoped_refptr<base::SingleThreadTaskRunner>
-HeadlessBrowserImpl::BrowserMainThread() const {
+HeadlessBrowserImpl::BrowserFileThread() const {
   return content::BrowserThread::GetTaskRunnerForThread(
-      content::BrowserThread::UI);
+      content::BrowserThread::FILE);
 }
 
 scoped_refptr<base::SingleThreadTaskRunner>
-HeadlessBrowserImpl::BrowserFileThread() const {
+HeadlessBrowserImpl::BrowserIoThread() const {
   return content::BrowserThread::GetTaskRunnerForThread(
-      content::BrowserThread::FILE);
+      content::BrowserThread::IO);
+}
+
+scoped_refptr<base::SingleThreadTaskRunner>
+HeadlessBrowserImpl::BrowserMainThread() const {
+  return content::BrowserThread::GetTaskRunnerForThread(
+      content::BrowserThread::UI);
 }
 
 void HeadlessBrowserImpl::Shutdown() {