Make --stress-background-compile a V8 flag rather than a d8 flag, so
that it also tests unittests/cctests.

Now, with this flag, every top-level script compile (that fulfills a
couple of restrictions) will be both main-thread and background-thread
compiled, taking the result of the background compile. In the future,
we'll probably want to verify that the two results are equivalent.

One of the necessary changes to allow tests to pass was to introduce a
concept of a "temporary" script (with a temporary script id), which
doesn't get added to the script list. This is to avoid the main-thread
compile part of the stress-test having a debugger-visible side-effect,
e.g. in tests that enumerate scripts. We can't just create new ids for
such scripts, as then script-id expectation files no longer match.

Bug: chromium:1011762
Change-Id: I500bbf2cabea762e69aca3dbae247daae71192cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2120541
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67332}