[headless] Make browser service manifest overlay configurable.

headless/test/headless_test_launcher.cc

Index: headless/test/headless_test_launcher.cc
diff --git a/headless/test/headless_test_launcher.cc b/headless/test/headless_test_launcher.cc
index da4550052cd018b321a97118501b4dcb73544369..57015f4f33bc0e01bfbe2cbb91db34876e62bc85 100644
--- a/headless/test/headless_test_launcher.cc
+++ b/headless/test/headless_test_launcher.cc
@@ -47,7 +47,8 @@ class HeadlessTestLauncherDelegate : public content::TestLauncherDelegate {
 
  protected:
   content::ContentMainDelegate* CreateContentMainDelegate() override {
-    // Use HeadlessBrowserTest::SetBrowserOptions to override these defaults.
+    // Use HeadlessBrowserTest::options() or HeadlessBrowserContextOptions to
+    // modify these defaults.
     HeadlessBrowser::Options::Builder options_builder;
     std::unique_ptr<HeadlessBrowserImpl> browser(
         new HeadlessBrowserImplForTest(options_builder.Build()));