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/Icon.cpp b/fpdfsdk/javascript/Icon.cpp
index cfce52f..778c1ed 100644
--- a/fpdfsdk/javascript/Icon.cpp
+++ b/fpdfsdk/javascript/Icon.cpp
@@ -44,7 +44,7 @@
   return m_swIconName;
 }
 
-bool Icon::name(IJS_EventContext* cc,
+bool Icon::name(CJS_Runtime* pRuntime,
                 CJS_PropValue& vp,
                 CFX_WideString& sError) {
   if (!vp.IsGetting())