port 1beb89f2 (r39827)

  original commit message:
  This patch simplifies code for speeding up marking and
  removes write barrier counter.

  The step size is now computed based in two parts:
  - bytes to mark in order to keep up with allocation,
  - bytes to mark in order to make progress.

BUG=

Review-Url: https://codereview.chromium.org/2407733003
Cr-Commit-Position: refs/heads/master@{#40116}