XFA: contention between FXJSE and FXJS over isolate data slots

This probably broke at 06b60021e when the FXJS slot moved to 0
from 1 unless explicitly overriden by the embedder, which conflicted
with the FXJSE_ usage of slot 0.

Also simplify some logic used to track global intialization of the
underling JS.

TEST=run_javascript_tests.py on XFA branch doesn't segv.
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1351173002 .
9 files changed