Merge to master: contention over isolate data slots

Work on this was first performed on the XFA branch, since
it has additional requirements (FXJSE layer) that needed
to be accomodated by the solution.

(cherry picked from commit ed7b2b50aa1744e0bc5a60bef12c61fa91d863b7)
Original Review URL: https://codereview.chromium.org/1351173002 .

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1354593004 .
3 files changed