Make v8 embedder data slot configurable

public/fpdf_formfill.h

Index: public/fpdf_formfill.h
diff --git a/public/fpdf_formfill.h b/public/fpdf_formfill.h
index 1cc5551e0bae45768826f9772efa7660701a7a84..5c568ae7d914792fe00770cf06fa11dc05090632 100644
--- a/public/fpdf_formfill.h
+++ b/public/fpdf_formfill.h
@@ -231,6 +231,11 @@ typedef struct _IPDF_JsPlatform
     *   pointer to the v8::Isolate to use, or NULL to force PDFium to create one.
     **/
     void*   m_isolate;
+
+    /**
+     *   The embedder data slot to use in the v8::Isolate.

[Tom Sepez, 2015/07/15 22:27:57]

nit: need a better comment here, as in what to pass if you're not sure, or why
you would change it, otherwise embedders might have trouble.

+     */
+    unsigned int m_v8EmbedderSlot;
 } IPDF_JSPLATFORM;
 
 // Flags for Cursor type