Fix and enable MSVC warning C4018 for base/.

base/files/file_enumerator_win.cc

Index: base/files/file_enumerator_win.cc
diff --git a/base/files/file_enumerator_win.cc b/base/files/file_enumerator_win.cc
index ae41a4600d31568ec66cc2d0d4705b86a48be171..90db7f5729efc624b76b0f328bfcfdac86793055 100644
--- a/base/files/file_enumerator_win.cc
+++ b/base/files/file_enumerator_win.cc
@@ -30,7 +30,8 @@ int64 FileEnumerator::FileInfo::GetSize() const {
   ULARGE_INTEGER size;
   size.HighPart = find_data_.nFileSizeHigh;
   size.LowPart = find_data_.nFileSizeLow;
-  DCHECK_LE(size.QuadPart, std::numeric_limits<int64>::max());
+  DCHECK_LE(size.QuadPart,
+            static_cast<ULONGLONG>(std::numeric_limits<int64>::max()));

[Nico, 2015/07/20 23:39:27]

Does std::numeric_limits<ULONGLONG>::max() not work?

[Peter Kasting, 2015/07/20 23:45:50]

That's not correct.  We're going to cast a ULONGLONG to an int64, so we want to
check that it's not over the int64 max.  Checking against ULONGLONG max would
tautologically be true.

[Nico, 2015/07/20 23:48:09]

Ah, I see.

   return static_cast<int64>(size.QuadPart);
 }