Rewrite 'worker.onerror' tests.

'fast/workers/worker-script-error.html' tests a variety of things, but
does so fairly inconsistently, and in a manner that's difficult to
parse, as the worker scripts are separate from the test itself. This
patch splits that test up into 8 separate tests, and uses our thrilling
Blob constructor to do some magical inlining to make things easier to
follow.

Review URL: https://chromiumcodereview.appspot.com/22530004

git-svn-id: svn://svn.chromium.org/blink/trunk@155693 bbb929c8-8fbe-4397-9dbb-9b2b20218538