Fix the issue that no response from the call back function of FFI_GetPlatform()

fpdfsdk/src/javascript/app.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "app.h"
 
 #include "Document.h"
 #include "JS_Context.h"
@@ skipping 199 matching lines @@
   }
 #endif  // PDF_ENABLE_XFA
 
   vp << JS_NUM_VIEWERVERSION;
   return TRUE;
 }
 
 FX_BOOL app::platform(IJS_Context* cc,
                       CJS_PropValue& vp,
                       CFX_WideString& sError) {
-  if (vp.IsGetting()) {
+  if (!vp.IsGetting())
+    return FALSE;
+  CPDFDoc_Environment* pEnv =
+      static_cast<CJS_Context*>(cc)->GetJSRuntime()->GetReaderApp();
+  if (!pEnv)
+    return FALSE;
+  CFX_WideString platfrom = pEnv->FFI_GetPlatform();
+  if (platfrom.IsEmpty())
     vp << JS_STR_PLATFORM;
-    return TRUE;
-  }
-
-  return FALSE;
+  else
+    vp << platfrom;
+  return TRUE;
 }
 
 FX_BOOL app::language(IJS_Context* cc,
                       CJS_PropValue& vp,
                       CFX_WideString& sError) {
   if (vp.IsGetting()) {
     vp << JS_STR_LANGUANGE;
     return TRUE;
   }
 
@@ skipping 578 matching lines @@
 FX_BOOL app::media(IJS_Context* cc, CJS_PropValue& vp, CFX_WideString& sError) {
   return FALSE;
 }
 
 FX_BOOL app::execDialog(IJS_Context* cc,
                         const std::vector<CJS_Value>& params,
                         CJS_Value& vRet,
                         CFX_WideString& sError) {
   return TRUE;
 }