Stop calling Persistent::uninitialize after blink::shutdown is called.

Stop calling Persistent::uninitialize after blink::shutdown is called.

This CL adds a short-term hack to prevent use-after-frees during a shutdown sequence.
Currently the following scenario can happen:

1) blink::shutdown() frees the underlying storage for persistent nodes.
2) ~MessageLoop() destructs some Chromium-side objects that hold Persistent.
It touches the underlying storage and crashes.

In middle term, we should entirely remove the shutdown sequence and get rid of the hack.

BUG=653688
Review-Url: https://codereview.chromium.org/2423743003
Cr-Commit-Position: refs/heads/master@{#425620}
(cherry picked from commit 10d68110da927986332bd753b17c08db0feac1c8)

[haraken, 2016-10-28 08:53:19]

Committed patchset #1 (id:1) to pending queue manually as
06c328da54fc69ba48cef2c3ce10a442dccf0eb0.