Bug 1598305, replace frame script in test browser_findbar.js with SpecialPowers.spawn, r=mikedeboer

Differential Revision: https://phabricator.services.mozilla.com/D61912

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

toolkit/content/tests/browser/browser_findbar.js

@@ -39,16 +39,15 @@ add_task(async function test_hotkey_event_propagation() {
   }
 
   // Stop propagation for all keyboard events.
-  let frameScript = () => {
-    const stopPropagation = e => e.stopImmediatePropagation();
+  await SpecialPowers.spawn(browser, [], () => {
+    const stopPropagation = e => {
+      e.stopImmediatePropagation();
+    };
     let window = content.document.defaultView;
-    window.removeEventListener("keydown", stopPropagation);
-    window.removeEventListener("keypress", stopPropagation);
-    window.removeEventListener("keyup", stopPropagation);
-  };
-
-  let mm = browser.messageManager;
-  mm.loadFrameScript("data:,(" + frameScript.toString() + ")();", false);
+    window.addEventListener("keydown", stopPropagation);
+    window.addEventListener("keypress", stopPropagation);
+    window.addEventListener("keyup", stopPropagation);
+  });
 
   // Checking if findbar still appears when any hotkey is pressed.
   for (let key of HOTKEYS) {