The flag is on by default, so we don't need to specify it. More
importantly, the tests are expected to work for any value of that flag.
So don't force the flag but use whatever the test variant chooses.

Note that in streaming-compile.js, the flag was accidentally specified
as '-async-compilation'. I also removed that one.

R=ahaas@chromium.org

Change-Id: Ifad31160d266dda38cdd9dd1d73dad69bd2c2f2c
Reviewed-on: https://chromium-review.googlesource.com/c/1325961Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57406}