This patch removes the code for speeding up marking.

Now the step size depends on allocated bytes and
invoked write barriers.

We also ensure that the step size is large enough to
justify the overhead of interrupting the generated code.

BUG=chromium:616434
LOG=NO

Review-Url: https://codereview.chromium.org/2290333002
Cr-Commit-Position: refs/heads/master@{#39067}