Bug 1926815 Remove inline event handler from fullscreen warning notification r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D226811

browser/base/content/browser-fullScreenAndPointerLock.js

@@ -305,6 +305,11 @@ var FullScreen = {
       "permissions.fullscreen.allowed"
     );
 
+    let notificationExitButton = document.getElementById(
+      "fullscreen-exit-button"
+    );
+    notificationExitButton.addEventListener("click", this.exitDomFullScreen);
+
     // Called when the Firefox window go into fullscreen.
     addEventListener("fullscreen", this, true);
 
@@ -400,6 +405,7 @@ var FullScreen = {
   },
 
   exitDomFullScreen() {
+    // Don't use `this` here. It does not reliably refer to this object.
     if (document.fullscreen) {
       document.exitFullscreen();
     }

browser/base/content/fullscreen-and-pointerlock.inc.xhtml

@@ -10,7 +10,6 @@
     <html:div class="pointerlockfswarning-generic-text"
               data-l10n-id="fullscreen-warning-no-domain"></html:div>
     <html:button id="fullscreen-exit-button"
-                 onclick="FullScreen.exitDomFullScreen();"
 #ifdef XP_MACOSX
                  data-l10n-id="fullscreen-exit-mac-button"
 #else