[headless] Make browser service manifest overlay configurable.

headless/test/headless_browser_test.h

Index: headless/test/headless_browser_test.h
diff --git a/headless/test/headless_browser_test.h b/headless/test/headless_browser_test.h
index 8b6cb75e52a6ad8750c9608feb96d26f65fca133..c787859eb5b84c5f4a03f41809cef22bae8c37b5 100644
--- a/headless/test/headless_browser_test.h
+++ b/headless/test/headless_browser_test.h
@@ -80,6 +80,10 @@ class HeadlessBrowserTest : public content::BrowserTestBase {
   // Returns the browser for the test.
   HeadlessBrowser* browser() const;
 
+  // Returns the options used by the browser. Modify with caution, since some
+  // options only take effect if they were set before browser creation.
+  HeadlessBrowser::Options* options() const;
+
  private:
   std::unique_ptr<base::RunLoop> run_loop_;