Merge to XFA: Don't pass null isolates to FXJS_ when we have a real isolate.

(cherry picked from commit 34b39e1d4fa0b16d47770b823230344356cc451a)
Original Review URL: https://codereview.chromium.org/1350703003 .

TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1342403005 .
6 files changed