The V8::IdleNotification will only return 'True' when the gc idle time handler
thinks there is no more GC which can be done. However, the gc idle task handler
can end up repeatedly making no progress (e.g., if it can't finalize a sweep)
which causes idle tasks to be repeatedly scheduled in Chrome which do nothing
but wake up Chrome. Fix this by returning Done if we can't make any progress
within an Idle Round.

BUG=chromium:470615
LOG=Y

Review URL: https://codereview.chromium.org/1042483002

Cr-Commit-Position: refs/heads/master@{#27529}