Allow creating non-thread-local Persistents that can be passed between threads.

Before this change all persistents must have been created and destroyed on the same thread.

This change introduces Persistent<T, GlobalPersistents> type. Such persistent can be safely allocated and destroyed on any thread.

BUG=

Review URL: https://codereview.chromium.org/169653003

git-svn-id: svn://svn.chromium.org/blink/trunk@167303 bbb929c8-8fbe-4397-9dbb-9b2b20218538