The current GC idle time handling heuristics are getting too complicated. Moreover, with longer idle time we are getting more full garbage collections. This CL shrinks the idle round window and reduces complexity in the case where we cause a full garbage collection.

BUG=chromium:468554
LOG=n

Review URL: https://codereview.chromium.org/1024043003

Cr-Commit-Position: refs/heads/master@{#27493}