Forcing finalization after reaching allocation limit regresses gc pause
time in benchmarks as we have to do a lot of non-incremental marking work.

This patch allows overshoot of the limit by some margin.

BUG=chromium:670675,chromium:671994
TBR=mlippautz@chromium.org

Review-Url: https://codereview.chromium.org/2554423005
Cr-Commit-Position: refs/heads/master@{#41625}