Pass v8::Isolate to PDFium at init time.

Move the external isolate and embedder slot from the
IPDF_JSPlatforms struct supplied at the
FPDFDOC_InitFormFillEnvironment() call time to arguments to
the FPDF_InitLibraryWithConfig() call.

This has several benefits:
-- Avoids the crash that could happen if multiple
FPDFDOC_InitFormFillEnvironmen() calls should happen to be
made by an embedder with different slot values.
-- Down the road, for XFA, there may be XFA but no FormFill
environment.

We support both forms for the time being, until the chrome
side catches up, at which point we will deprecate the old
way.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1367033002 .
9 files changed