This adds gc stress and scavenge stress to legacy gc fuzzer. Each
flag can be used in separation or combined.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:6972
Change-Id: Id03ef96bd42b77415dd201e7f78b3b42786dd3bc
Reviewed-on: https://chromium-review.googlesource.com/874452
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50708}