--fuzzer-gc-analysis prints allocations count without need for
predictable mode.

--random_gc_interval is similar to --gc-interval with 2 differences:
1) Causes gc every random(0, X) allocations instead of constant X.
2) Doesn't update the allocation timeout when there is a GC for
different reason than timeout equals to 0.

Bug: v8:6972
Change-Id: I07ad935fc264a61069255c7358b4fcbe42bfb17a
Reviewed-on: https://chromium-review.googlesource.com/815214
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50456}