Reduce signed/unsigned comparison warnings

core/fxcrt/include/fx_ext.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 CORE_FXCRT_INCLUDE_FX_EXT_H_
 #define CORE_FXCRT_INCLUDE_FX_EXT_H_
 
 #include <cctype>
 #include <cwctype>
 
 #include "core/fxcrt/include/fx_basic.h"
 
+#define FX_INVALID_OFFSET static_cast<uint32_t>(-1)
+
 // TODO(thestig) Using unique_ptr with ReleaseDeleter is still not ideal.
 // Come up or wait for something better. This appears in this file rather
 // than fx_stream.h due to include ordering restrictions.
 using ScopedFileStream =
     std::unique_ptr<IFX_FileStream, ReleaseDeleter<IFX_FileStream>>;
 
 FX_FLOAT FXSYS_tan(FX_FLOAT a);
 FX_FLOAT FXSYS_logb(FX_FLOAT b, FX_FLOAT x);
 FX_FLOAT FXSYS_strtof(const FX_CHAR* pcsStr,
                       int32_t iLength = -1,
@@ skipping 100 matching lines @@
           j -= gap;
         }
         pArray[j + gap] = v1;
       }
       gap >>= 1;
     }
   }
 };
 
 #endif  // CORE_FXCRT_INCLUDE_FX_EXT_H_