Fix gr-hovercard-behavior under Firefox

Firefox doesn't implement toElement/fromElement (they're not in the
spec, and were implemented by Chrome apparently for compatibility with
IE), but does implement target and relatedTarget, which are part of
the UI Events spec
(https://w3c.github.io/uievents/#interface-mouseevent).

Bug: Issue 13175
Change-Id: I9eeb26c032a38de9d7185749373c7982c796acb2
diff --git a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
index 04c3166..3d5937b 100644
--- a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
+++ b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
@@ -209,8 +209,8 @@
     // from the hovercard but now hovering over the target (to stop an annoying
     // flicker effect), just return.
     if (opt_e) {
-      if (opt_e.toElement === this ||
-          (opt_e.fromElement === this && opt_e.toElement === this._target)) {
+      if (opt_e.relatedTarget === this ||
+          (opt_e.target === this && opt_e.relatedTarget === this._target)) {
         return;
       }
     }