Remove V8PerIsolateData::current()

V8PerIsolateData::current() is not good because it calls v8::Isolate::GetCurrent(). This CL passes Isolate* around and remove V8PerIsolateData::current().

BUG=357144

Review URL: https://codereview.chromium.org/222003004

git-svn-id: svn://svn.chromium.org/blink/trunk@170831 bbb929c8-8fbe-4397-9dbb-9b2b20218538