Save 8 bytes in each CFX_ByteString/WideString (on "LP64" platforms).

core/src/fxcrt/fx_basic_wstring.cpp

Index: core/src/fxcrt/fx_basic_wstring.cpp
diff --git a/core/src/fxcrt/fx_basic_wstring.cpp b/core/src/fxcrt/fx_basic_wstring.cpp
index 42a7ad72a6182225c27c7fcc878b08a1e072ebe5..742f249e37d5f523b3c3169e18bf7a8a3d4f41fb 100644
--- a/core/src/fxcrt/fx_basic_wstring.cpp
+++ b/core/src/fxcrt/fx_basic_wstring.cpp
@@ -4,6 +4,8 @@
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
+#include <stddef.h>  // For offsetof().
+
 #include "../../include/fxcrt/fx_basic.h"
 #include "../../../third_party/base/numerics/safe_math.h"
 
@@ -15,7 +17,8 @@ static CFX_StringDataW* FX_AllocStringW(int nLen)
         return NULL;
     }
 
-    int overhead = 3 * sizeof(long) + sizeof(FX_WCHAR);  // +WCHAR is for NUL.
+    // Fixed portion of header plus a NUL wide char not in m_nAllocLength.
+    int overhead = offsetof(CFX_StringDataW, m_String) + sizeof(FX_WCHAR);
     pdfium::base::CheckedNumeric<int> iSize = nLen;
     iSize *= sizeof(FX_WCHAR);
     iSize += overhead;