Make UTF8ToWide take a StringPiece, to avoid copying to a std::string.

base/string_util_icu.cc

Index: base/string_util_icu.cc
diff --git a/base/string_util_icu.cc b/base/string_util_icu.cc
index 9036ce69f868311148b2d9623ead8adcb8323e2e..78b8de165adf04075bd1d1d0165d0dfad54763bf 100644
--- a/base/string_util_icu.cc
+++ b/base/string_util_icu.cc
@@ -198,7 +198,7 @@ bool WideToUTF8(const wchar_t* src, size_t src_len, std::string* output) {
   return ConvertUnicode<wchar_t, std::string>(src, src_len, output);
 }
 
-std::wstring UTF8ToWide(const std::string& utf8) {
+std::wstring UTF8ToWide(const StringPiece& utf8) {
   std::wstring ret;
   if (utf8.empty())
     return ret;