Introduce kPerIsolateDataIndex and tidy JS_Define.h

Follow-up from https://codereview.chromium.org/1366053003/

- use kPerIsolateDataIndex rather than magic constant 1.
- make a helper function for common code in JS_Define.h
- remove dead prototypes missed in earlier CL.
- fxjs_v8 can't include generic fpdfsdk includes (layering).

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1367813003 .