Sample new space allocation throughput at scavenge and at idle notificatioon.

This will allow better estimation of mutator idleness for switching between
latency and memory modes in idle notification handler.

BUG=chromium:486005
LOG=NO
TEST=cctest/test-heap/NewSpaceAllocationThroughput

Review URL: https://codereview.chromium.org/1125193005

Cr-Commit-Position: refs/heads/master@{#28537}