As required by C++11, this CL changes the zone allocator to be able to
construct and destroy arbitrary types, and accept arbitrary arguments
for construct, passing them via perfect forwarding.
I also change some push_back to emplace_back. Some of those did not
compile before.

R=ishell@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2646873004
Cr-Commit-Position: refs/heads/master@{#42597}