Bug 1504157 - make onsize event triggered once; r=smaug

avoid the condition been checked twice.

Differential Revision: https://phabricator.services.mozilla.com/D18961

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

browser/components/resistfingerprinting/test/browser/head.js

@@ -189,14 +189,14 @@ async function testWindowSizeSetting(aBrowser, aSettingWidth, aSettingHeight,
 
       // Test inner/outerWidth.
       await new Promise(resolve => {
-        win.onresize = () => {
+        win.addEventListener("resize", () => {
           is(win.screen.width, input.targetWidth,
             "The screen.width has a correct rounded value");
           is(win.innerWidth, input.targetWidth,
             "The window.innerWidth has a correct rounded value");
 
           resolve();
-        };
+        }, { once: true });
 
         if (input.testOuter) {
           win.outerWidth = input.settingWidth;
@@ -207,14 +207,14 @@ async function testWindowSizeSetting(aBrowser, aSettingWidth, aSettingHeight,
 
       // Test inner/outerHeight.
       await new Promise(resolve => {
-        win.onresize = () => {
+        win.addEventListener("resize", () => {
           is(win.screen.height, input.targetHeight,
             "The screen.height has a correct rounded value");
           is(win.innerHeight, input.targetHeight,
             "The window.innerHeight has a correct rounded value");
 
           resolve();
-        };
+        }, { once: true });
 
         if (input.testOuter) {
           win.outerHeight = input.settingHeight;