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/fpdfsave.cpp b/fpdfsdk/fpdfsave.cpp
index 364f4d2..9cdc714 100644
--- a/fpdfsdk/fpdfsave.cpp
+++ b/fpdfsdk/fpdfsave.cpp
@@ -36,7 +36,7 @@
 #include <ctime>
 #endif
 
-class CFX_IFileWrite final : public IFX_StreamWrite {
+class CFX_IFileWrite final : public IFX_WriteStream {
  public:
   CFX_IFileWrite();
   FX_BOOL Init(FPDF_FILEWRITE* pFileWriteStruct);