Reduce usage of v8::Isolate::GetCurrent() in NPV8Object

Reduce usage of v8::Isolate::GetCurrent() in NPV8Object and pass the isolate
around instead.

R=haraken
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/23537048

git-svn-id: svn://svn.chromium.org/blink/trunk@157889 bbb929c8-8fbe-4397-9dbb-9b2b20218538