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=chromium:616434, chromium:646139, chromium:644819
LOG=NO

Review-Url: https://codereview.chromium.org/2359903002
Cr-Commit-Position: refs/heads/master@{#39827}