First initialize the v8::Isolate on workers before passing it to gin

Gin will try to create v8::FunctionTemplates which would implictely
initialize the isolate.

BUG=321631
R=marja@chromium.org

Review URL: https://codereview.chromium.org/102853003

git-svn-id: svn://svn.chromium.org/blink/trunk@163799 bbb929c8-8fbe-4397-9dbb-9b2b20218538