Convert Field to use CPDFSDK_FormFillEnvironment

fpdfsdk/cpdfsdk_formfillenvironment.h

Index: fpdfsdk/cpdfsdk_formfillenvironment.h
diff --git a/fpdfsdk/cpdfsdk_formfillenvironment.h b/fpdfsdk/cpdfsdk_formfillenvironment.h
index e92537eee128cb4ae1bf4a200b701648f0e3f2a6..e5a693288f54256ad47f98c6b791ffc6ae0ce568 100644
--- a/fpdfsdk/cpdfsdk_formfillenvironment.h
+++ b/fpdfsdk/cpdfsdk_formfillenvironment.h
@@ -25,7 +25,8 @@ class CPDFSDK_AnnotHandlerMgr;
 class CPDFSDK_Document;
 class IJS_Runtime;
 
-class CPDFSDK_FormFillEnvironment final {
+class CPDFSDK_FormFillEnvironment
+    : public CFX_Observable<CPDFSDK_FormFillEnvironment> {
  public:
   CPDFSDK_FormFillEnvironment(UnderlyingDocumentType* pDoc,
                               FPDF_FORMFILLINFO* pFFinfo);