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..22ebc619b598c4778c0c3957e249e460f7cbda47 100644
--- a/public/fpdf_formfill.h
+++ b/public/fpdf_formfill.h
@@ -231,6 +231,13 @@ 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 to store PDFium's
+     *   per-isolate data. The value needs to be between 0 and
+     *   v8::Internals::kNumIsolateDataLots (exclusive).
+     */
+    unsigned int m_v8EmbedderSlot;
 } IPDF_JSPLATFORM;
 
 // Flags for Cursor type