Revert of Limit the usage of SharedPersistent to ScriptValue only (https://codereview.chromium.org/146523002/)

Reason for revert:
This change seems to be causing failures with plugins on windows 7 build:

http://build.chromium.org/p/chromium.webkit/builders/Win7%20%28dbg%29/builds/13319

The blame is 165731:165767 so this change looks most likely.


Original issue's description:
> We've replaced SharedPersistent with ScopedPersistent or ScriptValue before, but a couple of SharedPersistents are remaining. So this CL replaces them.
> 
> Ideally we want to remove SharedPersistent though.
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165751

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

Review URL: https://codereview.chromium.org/147333004

git-svn-id: svn://svn.chromium.org/blink/trunk@165802 bbb929c8-8fbe-4397-9dbb-9b2b20218538