Port d61a5c37

Original commit message:

    As a first step I uncommit the memory on the main thread. Also
    to measure impact and stability of that optimization. In a
    follow-up CL, the uncommitting should be moved on the concurrent thread.

R=jochen@chromium.org, hpayer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2041233003
Cr-Commit-Position: refs/heads/master@{#37332}