Bug 1520226 - Make GeckoView set permanentKey on <browser> elements for Marionette. r=snorp

Differential Revision: https://phabricator.services.mozilla.com/D17577

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

mobile/android/chrome/geckoview/geckoview.js

@@ -376,6 +376,9 @@ class ModuleInfo {
 
 function createBrowser() {
   const browser = window.browser = document.createElement("browser");
+  // Identify this `<browser>` element uniquely to Marionette, devtools, etc.
+  browser.permanentKey = {};
+
   browser.setAttribute("type", "content");
   browser.setAttribute("primary", "true");
   browser.setAttribute("flex", "1");