Push v8::Isolate into CFXJS_Engine class

Nearly all the "loose" functions in FXJS become methods on
the CFJXS_Engine.

This is the "missing link" wrt some layering violatons that
have been around forever. We can stop passing &m_ variables
from CJS_ down into FXJS Initialization as a result.

Review-Url: https://codereview.chromium.org/2245863002
diff --git a/fpdfsdk/javascript/JS_Object.cpp b/fpdfsdk/javascript/JS_Object.cpp
index d67ceeb..3de0ece 100644
--- a/fpdfsdk/javascript/JS_Object.cpp
+++ b/fpdfsdk/javascript/JS_Object.cpp
@@ -19,7 +19,7 @@
   CJS_Object* pJSObj = data.GetParameter();
   pJSObj->ExitInstance();
   delete pJSObj;
-  FXJS_FreePrivate(data.GetInternalField(0));
+  CFXJS_Engine::FreeObjectPrivate(data.GetInternalField(0));
 }
 
 void DisposeObject(const v8::WeakCallbackInfo<CJS_Object>& data) {