Use StringPiece for UTF string conversions.

base/strings/utf_string_conversions.h

Index: base/strings/utf_string_conversions.h
diff --git a/base/strings/utf_string_conversions.h b/base/strings/utf_string_conversions.h
index 3461aa491df0abb9b08be5dee91eb6f83d9997a1..9b27f3cb56bef507100a3e45162e0539032b3548 100644
--- a/base/strings/utf_string_conversions.h
+++ b/base/strings/utf_string_conversions.h
@@ -33,10 +33,9 @@ BASE_EXPORT bool UTF16ToWide(const char16* src, size_t src_len,
                              std::wstring* output);
 BASE_EXPORT std::wstring UTF16ToWide(const string16& utf16);
 
-BASE_EXPORT bool UTF8ToUTF16(const char* src, size_t src_len, string16* output);
+BASE_EXPORT bool UTF8ToUTF16(const StringPiece& utf8, string16* output);
 BASE_EXPORT string16 UTF8ToUTF16(const StringPiece& utf8);
-BASE_EXPORT bool UTF16ToUTF8(const char16* src, size_t src_len,
-                             std::string* output);
+BASE_EXPORT bool UTF16ToUTF8(const StringPiece16& utf16, std::string* output);
 BASE_EXPORT std::string UTF16ToUTF8(const string16& utf16);
 
 // We are trying to get rid of wstring as much as possible, but it's too big