Use override in more classes in fpdfsdk/

fpdfsdk/src/fpdfsave.cpp

Index: fpdfsdk/src/fpdfsave.cpp
diff --git a/fpdfsdk/src/fpdfsave.cpp b/fpdfsdk/src/fpdfsave.cpp
index 54c9253d2e4ed20e4b4efb97cc6f18445d2903ce..36553092eeae05e6402fb60a1da513695d7361bc 100644
--- a/fpdfsdk/src/fpdfsave.cpp
+++ b/fpdfsdk/src/fpdfsave.cpp
@@ -18,10 +18,12 @@ class CFX_IFileWrite final : public IFX_StreamWrite {
  public:
   CFX_IFileWrite();
   FX_BOOL Init(FPDF_FILEWRITE* pFileWriteStruct);
-  virtual FX_BOOL WriteBlock(const void* pData, size_t size) override;
-  virtual void Release() override {}
+  FX_BOOL WriteBlock(const void* pData, size_t size) override;
+  void Release() override;
 
  protected:
+  ~CFX_IFileWrite() override {}
+
   FPDF_FILEWRITE* m_pFileWriteStruct;
 };
 
@@ -45,6 +47,10 @@ FX_BOOL CFX_IFileWrite::WriteBlock(const void* pData, size_t size) {
   return TRUE;
 }
 
+void CFX_IFileWrite::Release() {
+  delete this;
+}
+
 FPDF_BOOL _FPDF_Doc_Save(FPDF_DOCUMENT document,
                          FPDF_FILEWRITE* pFileWrite,
                          FPDF_DWORD flags,
@@ -70,7 +76,7 @@ FPDF_BOOL _FPDF_Doc_Save(FPDF_DOCUMENT document,
   pStreamWrite = new CFX_IFileWrite;
   pStreamWrite->Init(pFileWrite);
   bRet = FileMaker.Create(pStreamWrite, flags);
-  delete pStreamWrite;
+  pStreamWrite->Release();
   return bRet;
 }