Add WTF::getPtr for Persistent pointers.

There is no WTF::getPtr for Persistent pointers.
We use the function in V8ValueTraits<T>::toV8Value.

BUG=NONE

Review URL: https://codereview.chromium.org/399843003

git-svn-id: svn://svn.chromium.org/blink/trunk@178348 bbb929c8-8fbe-4397-9dbb-9b2b20218538