Use ScopedPersistent for persistent handles in V8PerContextData

Using ScopedPersistent is safer, since ScopedPersistent will be automaticallly
destructed when the scope is exited.

(Another motivation is that I want to replace all raw persistent handles
with ScopedPersistent/UnsafePersistent in order to verify that persistent
handles don't leak among worlds.)

Review URL: https://codereview.chromium.org/149263005

git-svn-id: svn://svn.chromium.org/blink/trunk@166569 bbb929c8-8fbe-4397-9dbb-9b2b20218538