Merge to XFA: Allow compiling PDFium without V8.

Original Review URL: https://codereview.chromium.org/1395733006 .
(cherry picked from commit f1c713663192368d26031a4caed1f9705f4510af)

Conflicts:
  BUILD.gn
  fpdfsdk/src/fpdfview.cpp
  fpdfsdk/src/javascript/JS_Runtime.cpp
  pdfium.gyp
  samples/BUILD.gn
  samples/samples.gyp

BUG=pdfium:211
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1393833006 .
14 files changed