Bug 1438814 - Stop any further transitions creation once after we got a transitionend for the width transition. r=birtles

The transition property in this test is shorter than the period of chaging
parent viewport height so that it's possible that a new transition happens
after a transitionend event was dispatched.  The new transition will reduce
the element width (i.e. it's opposite direction of the first transition),
so the transitioned yellow box was smaller than the expected result.

Differential Revision: https://phabricator.services.mozilla.com/D4183

testing/web-platform/tests/css/css-values/support/vh_not_refreshing_on_chrome_iframe.html

@@ -80,6 +80,8 @@
 
 	addEventListener('transitionend', event => {
 		if (event.propertyName == 'width') {
+			// Stop any further transitons.
+			testBoxWithTransition.style.transitionProperty = 'none';
 			parent.postMessage('testBoxWithTransition', '*');
 		}
 	}, false);