Adding properties to prototypes is faster when we don't force their
maps into fast mode yet. Once a prototype shows up in the IC system,
its setup phase is likely over, and it makes sense to transition it
to fast properties.
This patch speeds up the microbenchmark in the bug by 20x.
Octane-Typescript sees a 3% improvement.

BUG=chromium:607010

Review-Url: https://codereview.chromium.org/2036493006
Cr-Commit-Position: refs/heads/master@{#36828}