Teach the fuzzer about the new DotAll flag.

Bug: v8:6612
Change-Id: I92d6bfd920f5daef6733b1c547063ede718ecc8f
Reviewed-on: https://chromium-review.googlesource.com/832748Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50188}