Remove the need for NPObjectWrapper around the window script object.

The V8 ScriptController now forces internal window script object resources to be released by Dispose()ing of the window script NPObject's underlying V8 object, rather than force-deallocating the NPObject. 

As a result, the NPObjectWrapper, used to work-around problems with plugins touching the window script object during teardown, after it has been deallocated, is no longer required. 

This is a re-land of https://src.chromium.org/viewvc/blink?revision=121610&view=revision which was originally reverted due to tickling NPObject ownership tracking bugs into crashing DRT in layout tests. Since Chrome & content-shell run NPAPI out-of-process, this should no longer be a problem.

BUG=152006

Review URL: https://chromiumcodereview.appspot.com/18859011

git-svn-id: svn://svn.chromium.org/blink/trunk@154201 bbb929c8-8fbe-4397-9dbb-9b2b20218538