Translate: Move label parsing logic into common code.

app/l10n_util.h

Index: app/l10n_util.h
diff --git a/app/l10n_util.h b/app/l10n_util.h
index 8ed7596e2ca8be6e676c98fc32e73a5b3fb64dc0..66b9d233554e6c4116efa1d77b691e01cb9a58b5 100644
--- a/app/l10n_util.h
+++ b/app/l10n_util.h
@@ -155,6 +155,10 @@ std::wstring GetStringF(int message_id,
                         const std::wstring& a,
                         const std::wstring& b,
                         std::vector<size_t>* offsets);
+string16 GetStringFUTF16(int message_id,
+                        const string16& a,
+                        const string16& b,
+                        std::vector<size_t>* offsets);
 
 // Convenience formatters for a single number.
 std::wstring GetStringF(int message_id, int a);