Explicitly test same/cross-origin importScripts() effect on worker.onerror.
We currently have a single test that checks behavior when calling
importScripts() on a URL that redirects from same-origin to cross-origin.
This patch adds tests for cross-origin and same-origin scripts, and updates
the tests to use js-test-pre.js.
Currently we have some weird behavior for cross-origin scripts. The
message is sanitized, the URL is set to the worker JS file, and the line
number is the call to 'importScripts'.
Firefox has different weird behavior: the message is unsanitized, but
the URL and line number are emptied.
I think it's reasonable to leave the expectations as they are, but our
eventual goal should be to sanitize these errors in the same way that we
do normal cross-origin messages.