Bug 1596800 - Remove document.getBindingParent usage from PluginChild.jsm. r=mconley

It wants to get the containing shadow host of the target to get to the plugin.
Do that explicitly.

Depends on D53337

Differential Revision: https://phabricator.services.mozilla.com/D53338

--HG--
extra : moz-landing-system : lando

browser/actors/PluginChild.jsm

@@ -707,8 +707,7 @@ class PluginChild extends JSWindowActorChild {
   }
 
   onOverlayClick(event) {
-    let document = event.target.ownerDocument;
-    let plugin = document.getBindingParent(event.target);
+    let plugin = event.target.containingShadowRoot.host;
     let overlay = this.getPluginUI(plugin, "main");
     // Have to check that the target is not the link to update the plugin
     if (