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
BUG=

Review-Url: https://codereview.chromium.org/2245703002
Cr-Commit-Position: refs/heads/master@{#38654}