Currently it is possible to get into a cycle of
mark-compact -> memory reducer -> mark-compact -> memory reducer ...
where the memory reducer does not free memory.

This patch ensures that the memory reducer restarts only if the
committed memory increased by sufficient amount after the last run.

BUG=

Review-Url: https://chromiumcodereview.appspot.com/2433933005
Cr-Commit-Position: refs/heads/master@{#40457}