Rewrite the 'window.onerror' tests.

The tests are currently duplicating work all over the place, and are
difficult to update. This patch tries to bring some consistency to the
way the feature is tested in the hopes of making future improvements
simpler to effectively test.

R=ch.dumez@sisa.samsung.com

Review URL: https://chromiumcodereview.appspot.com/19962008

git-svn-id: svn://svn.chromium.org/blink/trunk@154910 bbb929c8-8fbe-4397-9dbb-9b2b20218538