A fast promotion mode that works solely on GC heuristics without
requiring compiler or other profiler support by implementing zero-copy 
evacuation for new space.

- Once a threshold of survived bytes in the Scavenger is reached the
  mode is activated.
- In fast promotion mode all pages are moved to old space instead of
  performing a Scavenge.
- The inevitable upcoming full MC determines whether the decision
  whether the mode should stay on or be turned off based on the young
  generation survival rate.

BUG=chromium:693413

Change-Id: Ifdf296092a9bac609f9dcdfb47a24046f3093745
Reviewed-on: https://chromium-review.googlesource.com/442560
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43276}