Drop Isolate::GetCurrent() call from V8PerIsolateData

BUG=none
TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/227733003

git-svn-id: svn://svn.chromium.org/blink/trunk@170988 bbb929c8-8fbe-4397-9dbb-9b2b20218538