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_Runtime_Stub.cpp b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
index 109be0a..98c641f 100644
--- a/fpdfsdk/javascript/JS_Runtime_Stub.cpp
+++ b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
@@ -147,7 +147,8 @@
   }
 #endif  // PDF_ENABLE_XFA
 
-  int Execute(const CFX_WideString& script, CFX_WideString* info) override {
+  int ExecuteScript(const CFX_WideString& script,
+                    CFX_WideString* info) override {
     return 0;
   }