Add CJS_Field::GetFirstFormField().

Its what many CJS_Field methods really want to do.

Change-Id: I18cf61a15be7b783b795c61a5c26c42ef5d82127
Reviewed-on: https://pdfium-review.googlesource.com/37711
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fxjs/cjs_field.h b/fxjs/cjs_field.h
index 52ddbf7..1263f0b 100644
--- a/fxjs/cjs_field.h
+++ b/fxjs/cjs_field.h
@@ -405,6 +405,7 @@
                       int& iControlNo);
   std::vector<CPDF_FormField*> GetFormFields(
       const WideString& csFieldName) const;
+  CPDF_FormField* GetFirstFormField() const;
   CPDF_FormControl* GetSmartFieldControl(CPDF_FormField* pFormField);
   bool ValueIsOccur(CPDF_FormField* pFormField, WideString csOptLabel);