Add 'self.onerror' to Worker tests.

We're currently not testing 'self.onerror' for same-domain or cross-domain
calls to 'importScript'. This patch adds tests for this functionality, but makes
no functional changes.

R=ch.dumez@sisa.samsung.com

Review URL: https://codereview.chromium.org/19605005

git-svn-id: svn://svn.chromium.org/blink/trunk@154914 bbb929c8-8fbe-4397-9dbb-9b2b20218538