Previously we only supported strings and not filenames.  This
changes the default to filename and adds a new `type: string` which can
be passed `options` to allow for strings to be passed in test code.

See: https://developer.mozilla.org/en-US/docs/Web/API/Worker/Worker

Bug: v8:8020
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ie8818885c5c5c071b6614852322cb45aeb01a647
Reviewed-on: https://chromium-review.googlesource.com/1185980
Commit-Queue: Sam Clegg <sbc@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56056}