Code change to avoid signed/unsigned mismatch warnings

core/fxcrt/include/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 CORE_FXCRT_INCLUDE_FX_STRING_H_
 #define CORE_FXCRT_INCLUDE_FX_STRING_H_
 
 #include <stdint.h>  // For intptr_t.
@@ skipping 116 matching lines @@
 
  private:
   void* operator new(size_t) throw() { return NULL; }
 };
 inline bool operator==(const char* lhs, const CFX_ByteStringC& rhs) {
   return rhs == lhs;
 }
 inline bool operator!=(const char* lhs, const CFX_ByteStringC& rhs) {
   return rhs != lhs;
 }
-#define FXBSTR_ID(c1, c2, c3, c4) ((c1 << 24) | (c2 << 16) | (c3 << 8) | (c4))
+#define FXBSTR_ID(c1, c2, c3, c4)                                      \
+  (((uint32_t)c1 << 24) | ((uint32_t)c2 << 16) | ((uint32_t)c3 << 8) | \
+   ((uint32_t)c4))
 
 // 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(nullptr) {}
 
   // Copy constructor.
@@ skipping 662 matching lines @@
 }
 
 FX_FLOAT FX_atof(const CFX_ByteStringC& str);
 inline FX_FLOAT FX_atof(const CFX_WideStringC& wsStr) {
   return FX_atof(FX_UTF8Encode(wsStr.GetPtr(), wsStr.GetLength()));
 }
 void FX_atonum(const CFX_ByteStringC& str, FX_BOOL& bInteger, void* pData);
 FX_STRSIZE FX_ftoa(FX_FLOAT f, FX_CHAR* buf);
 
 #endif  // CORE_FXCRT_INCLUDE_FX_STRING_H_