V8 can use this metter to better estimate the amount of work that is still to be
done by the embedder. Supposed to be used to decide whether incremental marking
should be finalized or not.

BUG=chromium:468240
R=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2291613002
Cr-Commit-Position: refs/heads/master@{#38985}