Fix flaky test lang-mapped-to-webkit-locale-dynamic.xhtml

When the test is moved to run as an onload handler, the flakiness is
fixed locally.

It's not clear to me the true cause of the flakiness. It seems to be
some difference between html and xhtml script loading, as the flakiness
could not be reproduced on an html version of the test. There may also
be a real bug in getComputedStyle using a cached style.

WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=98087

Review URL: https://chromiumcodereview.appspot.com/23745006

git-svn-id: svn://svn.chromium.org/blink/trunk@157108 bbb929c8-8fbe-4397-9dbb-9b2b20218538