Bug 1631735 Part 3: Update test expectations to note macOS transitions are async. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D170841

widget/tests/test_sizemode_events.xhtml

@@ -24,8 +24,9 @@ const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 let gWindow = null;
 
 const kIsLinux = navigator.platform.includes("Lin");
-// On Linux sizemode changes might be async.
-const kAsyncChanges = kIsLinux;
+const kIsMacOS = navigator.platform.includes("Mac");
+// On Linux and macOS sizemode changes might be async.
+const kAsyncChanges = kIsLinux || kIsMacOS;
 
 let gSizeModeDidChange = false;
 let gSizeModeDidChangeTo = 0;