Tests: Don't test synchronous XHR on unload in Chrome

Chrome 78 dropped support for synchronous XHR requests inside of
beforeunload, unload, pagehide, and visibilitychange event handlers.
See https://bugs.chromium.org/p/chromium/issues/detail?id=952452

Closes gh-4536

(cherry picked from commit c5b48c8)

test/unit/ajax.js

@@ -2085,14 +2085,19 @@ if ( typeof window.ArrayBuffer === "undefined" || typeof new XMLHttpRequest().re
 		};
 	} );
 
-	testIframe(
-		"#14379 - jQuery.ajax() on unload",
-		"ajax/onunload.html",
-		function( assert, jQuery, window, document, status ) {
-			assert.expect( 1 );
-			assert.strictEqual( status, "success", "Request completed" );
-		}
-	);
+	// Chrome 78 dropped support for synchronous XHR requests inside of
+	// beforeunload, unload, pagehide, and visibilitychange event handlers.
+	// See https://bugs.chromium.org/p/chromium/issues/detail?id=952452
+	if ( !/chrome/i.test( navigator.userAgent ) ) {
+		testIframe(
+			"#14379 - jQuery.ajax() on unload",
+			"ajax/onunload.html",
+			function( assert, jQuery, window, document, status ) {
+				assert.expect( 1 );
+				assert.strictEqual( status, "success", "Request completed" );
+			}
+		);
+	}
 
 	ajaxTest( "#14683 - jQuery.ajax() - Exceptions thrown synchronously by xhr.send should be caught", 4, function( assert ) {
 		return [ {