We introduce, similar to regular heap, a hard and a soft limit for external memory.
- Upon reaching the hard limit we do a full GC. The hard limit is a a delta from
  the size of external memory at last GC.
- Upon reaching the soft limit we start incremental marking. Each further
  AdjustAmountOfExternalMemory will trigger a marking step. The step size depends
  on how far we are away from the hard limit. Further away means we have still
  some wiggle room and the step will be small. Being closer to the hard limit
  makes it more likely to trigger a full GC, so we increase the step size.

BUG=chromium:621829

Review-Url: https://codereview.chromium.org/2256853003
Cr-Commit-Position: refs/heads/master@{#39133}