Convert IJS_Runtime to CPDFSDK_FormFillEnvironment

This CL changes the use of CPDFSDK_Document to CPDFSDK_FormFillEnvironment.

Review-Url: https://codereview.chromium.org/2394313002
diff --git a/fpdfsdk/javascript/Field.cpp b/fpdfsdk/javascript/Field.cpp
index 51724c4..652f1e6 100644
--- a/fpdfsdk/javascript/Field.cpp
+++ b/fpdfsdk/javascript/Field.cpp
@@ -212,7 +212,7 @@
 FX_BOOL Field::AttachField(Document* pDocument,
                            const CFX_WideString& csFieldName) {
   m_pJSDoc = pDocument;
-  m_pFormFillEnv.Reset(pDocument->GetReaderDoc()->GetEnv());
+  m_pFormFillEnv.Reset(pDocument->GetFormFillEnv());
   m_bCanSet =
       m_pFormFillEnv->GetSDKDocument()->GetPermissions(FPDFPERM_FILL_FORM) ||
       m_pFormFillEnv->GetSDKDocument()->GetPermissions(FPDFPERM_ANNOT_FORM) ||