Revert of Use ScopedPersistent for persistent handles in V8PerContextData (https://codereview.chromium.org/149263005/)

Reason for revert:
This change is being blamed for many virtual/threaded/transitions timeouts on the bots. There is no obvious connection between these test timeouts and this patch, but this is the only change listed when the timeouts started to occur.

Some of the timeouts: http://goo.gl/6tX16t

Original issue's description:
> 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.)
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166569

TBR=jochen@chromium.org,dcarney@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/150283006

git-svn-id: svn://svn.chromium.org/blink/trunk@166577 bbb929c8-8fbe-4397-9dbb-9b2b20218538