Revert of Remove redundant activation threshold for incremental marking. (patchset #1 id:1 of https://codereview.chromium.org/1418293006/ )

Reason for revert:
Regression in thread_times.key_idle_power_cases (crbug.com/555153)

Original issue's description:
> Remove redundant activation threshold for incremental marking.
>
> The minimum allocation limit already enforces this constraint for normal GCs.
>
> GCs triggered by the memory reducer and external limit should work for all heap sizes.
>
> BUG=chromium:552305
> LOG=NO
>
> Committed: https://crrev.com/e28e4d5f52852e2f14facc37a02069b6bfc82c4e
> Cr-Commit-Position: refs/heads/master@{#31921}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:552305

Review URL: https://codereview.chromium.org/1456523002

Cr-Commit-Position: refs/heads/master@{#32038}