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.

BUG=159566
R=ch.dumez@sisa.samsung.com

Review URL: https://codereview.chromium.org/19514006

git-svn-id: svn://svn.chromium.org/blink/trunk@154839 bbb929c8-8fbe-4397-9dbb-9b2b20218538