Make code compile with clang_use_chrome_plugin (part III)

This change contains files in xfa/fxbarcode directory.
This is part of the efforts to make PDFium code compilable
by Clang chromium style plugins.

The changes are mainly the following:
-- move inline constructor/destructor of complex class/struct out-of-line;
-- add constructor/destructor of complex class/struct if not
 explicitly defined;
-- add explicit out-of-line copy constructor when needed;
-- move inline virtual functions out-of-line;
-- Properly mark virtual functions with 'override';
-- some minor cleanups;

BUG=pdfium:469

Review-Url: https://codereview.chromium.org/2067903002
diff --git a/xfa/fxbarcode/pdf417/BC_PDF417Writer.h b/xfa/fxbarcode/pdf417/BC_PDF417Writer.h
index 7f71959..e16c514 100644
--- a/xfa/fxbarcode/pdf417/BC_PDF417Writer.h
+++ b/xfa/fxbarcode/pdf417/BC_PDF417Writer.h
@@ -14,12 +14,16 @@
 class CBC_PDF417Writer : public CBC_TwoDimWriter {
  public:
   CBC_PDF417Writer();
-  virtual ~CBC_PDF417Writer();
+  ~CBC_PDF417Writer() override;
+
   uint8_t* Encode(const CFX_WideString& contents,
                   int32_t& outWidth,
                   int32_t& outHeight,
                   int32_t& e);
-  FX_BOOL SetErrorCorrectionLevel(int32_t level);
+
+  // CBC_TwoDimWriter
+  FX_BOOL SetErrorCorrectionLevel(int32_t level) override;
+
   void SetTruncated(FX_BOOL truncated);
 
  private: