Fix more disabled MSVC warnings, base/ edition.

base/strings/utf_string_conversion_utils.cc

Index: base/strings/utf_string_conversion_utils.cc
diff --git a/base/strings/utf_string_conversion_utils.cc b/base/strings/utf_string_conversion_utils.cc
index 09a003d1b9cedc509e9fb8ec86371c5a2e0aa31f..022c0dffd895ab7defaff9c482b7228b1e7e9b3a 100644
--- a/base/strings/utf_string_conversion_utils.cc
+++ b/base/strings/utf_string_conversion_utils.cc
@@ -71,7 +71,7 @@ bool ReadUnicodeCharacter(const wchar_t* src,
 size_t WriteUnicodeCharacter(uint32 code_point, std::string* output) {
   if (code_point <= 0x7f) {
     // Fast path the common case of one byte.
-    output->push_back(code_point);
+    output->push_back(static_cast<char>(code_point));
     return 1;
   }