Rename IFX_ stream names

It's been troubling for some time that an IFX_FileStream might
actually be an in-memory buffer with no backing file.

Review-Url: https://codereview.chromium.org/2443723002
diff --git a/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp b/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
index 7e7ee50..13b79ba 100644
--- a/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
+++ b/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
@@ -704,7 +704,7 @@
   return ret;
 }
 
-IFX_FileRead* CPDFXFA_DocEnvironment::OpenLinkedFile(
+IFX_SeekableReadStream* CPDFXFA_DocEnvironment::OpenLinkedFile(
     CXFA_FFDoc* hDoc,
     const CFX_WideString& wsLink) {
   CPDFSDK_FormFillEnvironment* pFormFillEnv = m_pDocument->GetFormFillEnv();