Refactoring to move unsafe handle accesses inside UnsafePersistent.

The perf-critical paths should already be migrated to direct return value
setting, so we can just create new local handles for the rest of the places.

Changes:
- Push the return value setting (and security checks) to UnsafePersistent.
- Use UnsafePersistent::persistent() instead of deprecatedHandle() whenever
possible (gets rid of reinterpret_casting between Persistent and Handle (the
classes are now unrelated).
- Remove DOMDataStore::get(), rename DOMDataStore::newLocal() to
DOMDataStore::get() (consistency; we used to have getWrapper + get which
return unsafe handles, now we have getWrapper + get which return local
handles).
- Remove UnsafePersistent friends.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/23147004

git-svn-id: svn://svn.chromium.org/blink/trunk@156465 bbb929c8-8fbe-4397-9dbb-9b2b20218538