Fix more disabled MSVC warnings, base/ edition.

base/strings/string_number_conversions.cc

Index: base/strings/string_number_conversions.cc
diff --git a/base/strings/string_number_conversions.cc b/base/strings/string_number_conversions.cc
index d6bd5c48070cfbba527be002734352284e10dcc8..642d24e9ada11ad61444fcadd81a0accddb0df65 100644
--- a/base/strings/string_number_conversions.cc
+++ b/base/strings/string_number_conversions.cc
@@ -103,7 +103,7 @@ template<typename CHAR, int BASE> class BaseCharToDigit<CHAR, BASE, true> {
  public:
   static bool Convert(CHAR c, uint8* digit) {
     if (c >= '0' && c < '0' + BASE) {
-      *digit = c - '0';
+      *digit = static_cast<uint8>(c - '0');
       return true;
     }
     return false;