Fix more disabled MSVC warnings, base/ edition.

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index e20bbf0a40c59f9440a23812ca7e8723dd2c9dec..32493aaaf252048380e690b7c543e65cd4f5d8f3 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -352,14 +352,14 @@ inline bool IsHexDigit(Char c) {
 }
 
 template <typename Char>
-inline Char HexDigitToInt(Char c) {
+inline char HexDigitToInt(Char c) {
   DCHECK(IsHexDigit(c));
   if (c >= '0' && c <= '9')
-    return c - '0';
+    return static_cast<char>(c - '0');
   if (c >= 'A' && c <= 'F')
-    return c - 'A' + 10;
+    return static_cast<char>(c - 'A' + 10);
   if (c >= 'a' && c <= 'f')
-    return c - 'a' + 10;
+    return static_cast<char>(c - 'a' + 10);
   return 0;
 }