Re-enable several MSVC warnings

core/fxcrt/fx_basic_wstring.cpp

Index: core/fxcrt/fx_basic_wstring.cpp
diff --git a/core/fxcrt/fx_basic_wstring.cpp b/core/fxcrt/fx_basic_wstring.cpp
index 95b761bd09e67a55b38ae5216ab5e555b8872a96..93570af04328fdb1639bf84bb32b36afe282b5ba 100644
--- a/core/fxcrt/fx_basic_wstring.cpp
+++ b/core/fxcrt/fx_basic_wstring.cpp
@@ -642,7 +642,8 @@ FX_STRSIZE CFX_WideString::Replace(const FX_WCHAR* lpszOld,
   FX_WCHAR* lpszEnd = m_pData->m_String + m_pData->m_nDataLength;
   FX_WCHAR* lpszTarget;
   {

[Tom Sepez, 2016/03/16 18:34:59]

nit: brace needed here?

[Wei Li, 2016/03/17 02:24:05]

Done.

-    while ((lpszTarget = (FX_WCHAR*)FXSYS_wcsstr(lpszStart, lpszOld)) &&
+    while ((lpszTarget = (FX_WCHAR*)FXSYS_wcsstr(lpszStart, lpszOld)) !=
+               nullptr &&
            lpszStart < lpszEnd) {

[Tom Sepez, 2016/03/16 18:34:59]

This should be fine.  Not sure we want to write it this way to make the compiler
happy.

[Wei Li, 2016/03/17 02:24:05]

For pdfium code, this warning is not that useful. But for some other cases,
people sometimes accidentally type "=" instead of "==". We have just a few (3 or
4?) cases like this to clean up. So I am fine following chromium code on this.  

       nCount++;
       lpszStart = lpszTarget + nSourceLen;
@@ -668,7 +669,7 @@ FX_STRSIZE CFX_WideString::Replace(const FX_WCHAR* lpszOld,
     lpszEnd = m_pData->m_String + std::max(m_pData->m_nDataLength, nNewLength);
     {
       while ((lpszTarget = (FX_WCHAR*)FXSYS_wcsstr(lpszStart, lpszOld)) !=
-                 NULL &&
+                 nullptr &&
              lpszStart < lpszEnd) {
         FX_STRSIZE nBalance =
             nOldLength -