Pass CJS_Runtime to JS callbacks.

This is much more convenient, since only a fraction of them
need an IJS_EventContext, which can be obtained from the
CJS_Runtime.

Make GetCurrentEventContext() specific to CJS_Runtime, and
return the concrete type.  This saves a lot of casting.

Change-Id: If79a3bcbf44de513f3caace153099234cc313d47
Reviewed-on: https://pdfium-review.googlesource.com/2793
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/javascript/JS_Runtime_Stub.cpp b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
index 12edc1b..dcd8ceb 100644
--- a/fpdfsdk/javascript/JS_Runtime_Stub.cpp
+++ b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
@@ -127,12 +127,9 @@
   IJS_EventContext* NewEventContext() override {
     if (!m_pContext)
       m_pContext = pdfium::MakeUnique<CJS_EventContextStub>();
-    return GetCurrentEventContext();
-  }
-
-  IJS_EventContext* GetCurrentEventContext() override {
     return m_pContext.get();
   }
+
   void ReleaseEventContext(IJS_EventContext* pContext) override {}
 
   CPDFSDK_FormFillEnvironment* GetFormFillEnv() const override {