Port: 1beb89f2

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.

R=mvstanton@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com, bjaideep@ca.ibm.com
BUG=

Review-Url: https://codereview.chromium.org/2378053003
Cr-Commit-Position: refs/heads/master@{#39876}