Merge to XFA: Introduce kPerIsolateDataIndex and tidy JS_Define.h

(cherry picked from commit d6d9dc6b9a45368abdf43477592ee9f6cdb6102e)
(cherry picked from commit cfff2f65aaec70247d020188bc68a0dc4fb34c3e)

Original Review URL: https://codereview.chromium.org/1372963003 .
Original Review URL: https://codereview.chromium.org/1367813003 .

TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/1370133007 .
5 files changed