Don't call persistent() on temporary UnsafePersistent object.

UnsafePersistent::persistent() method returns an address to its member. This shouldn't be called against temporary UnsafePersistent object as it may be destructed.

This patch ensures that UnsafePersistent is alive while its persistent() are used.

BUG=None

Review URL: https://codereview.chromium.org/27627004

git-svn-id: svn://svn.chromium.org/blink/trunk@159873 bbb929c8-8fbe-4397-9dbb-9b2b20218538