Make IJS_Runtime::AsCJSRuntime be XFA-only

Since it is only called from the XFA side.

Change-Id: I6e34b39bb0c28db9b367215fae0285acafa22c14
Reviewed-on: https://pdfium-review.googlesource.com/c/44810
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fxjs/cjs_runtimestub.h b/fxjs/cjs_runtimestub.h
index 779a675..b4bb2dd 100644
--- a/fxjs/cjs_runtimestub.h
+++ b/fxjs/cjs_runtimestub.h
@@ -21,12 +21,13 @@
   explicit CJS_RuntimeStub(CPDFSDK_FormFillEnvironment* pFormFillEnv);
   ~CJS_RuntimeStub() override;
 
-  CJS_Runtime* AsCJSRuntime() override;
+  // IJS_Runtime:
   IJS_EventContext* NewEventContext() override;
   void ReleaseEventContext(IJS_EventContext* pContext) override;
   CPDFSDK_FormFillEnvironment* GetFormFillEnv() const override;
 
 #ifdef PDF_ENABLE_XFA
+  CJS_Runtime* AsCJSRuntime() override;
   bool GetValueByNameFromGlobalObject(const ByteStringView&,
                                       CFXJSE_Value*) override;
   bool SetValueByNameInGlobalObject(const ByteStringView&,