[headless] Make browser service manifest overlay configurable.

headless/test/headless_browser_test.cc

Index: headless/test/headless_browser_test.cc
diff --git a/headless/test/headless_browser_test.cc b/headless/test/headless_browser_test.cc
index e259d9ba8440cca826704c19d905567826879d31..090a7045e4927819ae13e8528580dcc02b2bee52 100644
--- a/headless/test/headless_browser_test.cc
+++ b/headless/test/headless_browser_test.cc
@@ -151,6 +151,10 @@ HeadlessBrowser* HeadlessBrowserTest::browser() const {
   return HeadlessContentMainDelegate::GetInstance()->browser();
 }
 
+HeadlessBrowser::Options* HeadlessBrowserTest::options() const {
+  return HeadlessContentMainDelegate::GetInstance()->browser()->options();
+}
+
 bool HeadlessBrowserTest::WaitForLoad(HeadlessWebContents* web_contents) {
   SynchronousLoadObserver load_observer(this, web_contents);
   RunAsynchronousTest();