This patch
- extracts the logic of keeping track of allocated bytes
  from the actual incremental marking step.
- replaces OldSpaceStep with a check for incremental marking start.
- removes the force_marking parameter of AdvanceIncrementalMarking.

BUG=chromium:616434
LOG=NO

Review-Url: https://codereview.chromium.org/2304123003
Cr-Commit-Position: refs/heads/master@{#39213}