[ui] add a scrollbar when the overlay is too big

Change-Id: I2db375666fd13587e4f5335f38ddc8da1f67a351
diff --git a/rv-reviewers/rv-reviewers.html b/rv-reviewers/rv-reviewers.html
index dcf890d..f44aa6f 100644
--- a/rv-reviewers/rv-reviewers.html
+++ b/rv-reviewers/rv-reviewers.html
@@ -21,6 +21,7 @@
     <style include="shared-styles">
       #rvScreenOverlay {
         width: 50em;
+        overflow: auto;
       }
     </style>
     <gr-repo-command