Add mode to reduce memory usage in idle notification.

Add mode to reduce memory usage in idle notification.

While the mutator is active, the idle time handler optimizes for latency by doing only incremental steps and scavenges.

When the mutator becomes inactive, the idle time handler forces few incremental GCs to reclaim memory and then stops until mutator is active again.

BUG=460090
LOG=N

Review URL: https://codereview.chromium.org/1105293004

Cr-Commit-Position: refs/heads/master@{#28300}