Just print interesting idle notification events in --trace-idle-notification.

BUG=

Review URL: https://codereview.chromium.org/763063002

Cr-Commit-Position: refs/heads/master@{#25539}
Introduce a new growth criterion for the new space behind a flag

With this flag, we grow if more than 10% survived the last scavenge.

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/753543002

Cr-Commit-Position: refs/heads/master@{#25540}