Temporary implementation of GC fuzzer based on the deopt fuzzer.
It will be merged with the deopt fuzzer to create more general numfuzzer.

Webkit test suite updated to ignore trace_incremental_marking flag output.

Bug: v8:6917
Change-Id: I053ae32f3cd3f74ba8b71fc5bdfc7264f54dca11
Reviewed-on: https://chromium-review.googlesource.com/758437
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49296}