Merge to XFA: Refactor fxjs_v8 and add embeddertests for it.

(cherry picked from commit b17d62601b21dfce85718e08cfd0ffce3a45d74e)
(cherry picked from commit 09ed30750282bf56a92d0e646ab22c64bea81a36)
Manual edits:
  fpdfsdk/src/jsapi/fxjs_v8_embeddertest.cpp - add lockers.
  fppdfsdk/src/javascript/JS_Runtime.cpp - rework XFA init path.

Original Review URL: https://codereview.chromium.org/1338073002 .

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1348433002 .
8 files changed