Don't assert that no incremental marking happened during a non-incremental GC

If a scavenge is triggered during the GC (via a weak callback or the GC
prologue/epilogue), and another mark-compact is triggered (via the same
unfortunate code path), we end up doing some incremental steps.

BUG=v8:4048
R=dcarney@chromium.org,hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1096693011

Cr-Commit-Position: refs/heads/master@{#27956}