Convert the overlay scrollbar flag to use FeatureList

content/browser/site_per_process_browsertest.h

Index: content/browser/site_per_process_browsertest.h
diff --git a/content/browser/site_per_process_browsertest.h b/content/browser/site_per_process_browsertest.h
index fe8b84b0c8e96cc182a258cb7c21c60cfd8cf730..b37d734104f3ae362ea1a61cd8daf2eca9751bab 100644
--- a/content/browser/site_per_process_browsertest.h
+++ b/content/browser/site_per_process_browsertest.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/macros.h"
+#include "base/test/scoped_feature_list.h"
 #include "content/browser/web_contents/web_contents_impl.h"
 #include "content/public/test/content_browser_test.h"
 #include "content/shell/browser/shell.h"
@@ -34,6 +35,7 @@ class SitePerProcessBrowserTest : public ContentBrowserTest {
 
  private:
   FrameTreeVisualizer visualizer_;
+  base::test::ScopedFeatureList feature_list_;
 
   DISALLOW_COPY_AND_ASSIGN(SitePerProcessBrowserTest);
 };