Merge to XFA: Plumb in an externally created v8::Isolate

Origin Review URL:
- https://codereview.chromium.org/1234053003
- https://codereview.chromium.org/1241863004
- https://codereview.chromium.org/1235303003

R=tsepez@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1263963002 .
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index aa171af..7ecd9a2 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -427,7 +427,7 @@
 
   IPDF_JSPLATFORM platform_callbacks;
   memset(&platform_callbacks, '\0', sizeof(platform_callbacks));
-  platform_callbacks.version = 1;
+  platform_callbacks.version = 2;
   platform_callbacks.app_alert = ExampleAppAlert;
   platform_callbacks.Doc_gotoPage = ExampleDocGotoPage;