Backfill some FX String unit tests for == and !=.

core/include/fxcrt/fx_string.h

 // 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
 
 #ifndef _FX_STRING_H_
 #define _FX_STRING_H_
 
 #include <algorithm>
@@ skipping 160 matching lines @@
 struct CFX_StringData {
 
     long                m_nRefs;
 
     FX_STRSIZE  m_nDataLength;
 
     FX_STRSIZE  m_nAllocLength;
 
     FX_CHAR             m_String[1];
 };
-class CFX_ByteString 
+
+// A mutable string with shared buffers using copy-on-write semantics that
+// avoids the cost of std::string's iterator stability guarantees.
+class CFX_ByteString
 {
 public:
     typedef FX_CHAR value_type;
 
     CFX_ByteString()
     {
         m_pData = NULL;
     }
 
     CFX_ByteString(const CFX_ByteString& str);
@@ skipping 396 matching lines @@
 struct CFX_StringDataW {
 
     long                m_nRefs;
 
     FX_STRSIZE  m_nDataLength;
 
     FX_STRSIZE  m_nAllocLength;
 
     FX_WCHAR    m_String[1];
 };
-class CFX_WideString 
+
+// A mutable string with shared buffers using copy-on-write semantics that
+// avoids the cost of std::string's iterator stability guarantees.
+class CFX_WideString
 {
 public:
     typedef FX_WCHAR value_type;
 
     CFX_WideString()
     {
         m_pData = NULL;
     }
 
     CFX_WideString(const CFX_WideString& str);
@@ skipping 226 matching lines @@
 CFX_ByteString  FX_UTF8Encode(FX_LPCWSTR pwsStr, FX_STRSIZE len);
 inline CFX_ByteString   FX_UTF8Encode(FX_WSTR wsStr)
 {
     return FX_UTF8Encode(wsStr.GetPtr(), wsStr.GetLength());
 }
 inline CFX_ByteString   FX_UTF8Encode(const CFX_WideString &wsStr)
 {
     return FX_UTF8Encode(wsStr.c_str(), wsStr.GetLength());
 }
 #endif