Revert of Revert of Use ScopedPersistent for persistent handles in V8PerContextData (https://codereview.chromium.org/150283006/)

Reason for revert:
Haraken's original patch (https://src.chromium.org/viewvc/blink?view=rev&revision=166569) did not cause the timeouts we thought it did. I am relanding the patch that was incorrectly reverted.

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166577

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

Review URL: https://codereview.chromium.org/137213006

git-svn-id: svn://svn.chromium.org/blink/trunk@166578 bbb929c8-8fbe-4397-9dbb-9b2b20218538