When message have overflow Y content clip it.

This fix prevents stacking unicode from escaping the bounds of a
gr-message element and impacting the rest of the application.

Change-Id: I418ed07068a2b2af01ca4ff4512aa5764f600749
diff --git a/polygerrit-ui/app/elements/change/gr-message/gr-message.html b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
index b828327..a0f6fbe 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.html
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
@@ -37,6 +37,7 @@
         display: block;
         position: relative;
         cursor: pointer;
+        overflow-y: hidden;
       }
       :host(.expanded) {
         cursor: auto;