Change HMAC::Sign() to take base::StringPiece instead of string.

crypto/hmac.h

Index: crypto/hmac.h
diff --git a/crypto/hmac.h b/crypto/hmac.h
index c0706d8ebcf17d337ec16a293547dacdab7e0039..c98a0283fd7465d3b9ced8433642bb672f7632f0 100644
--- a/crypto/hmac.h
+++ b/crypto/hmac.h
@@ -13,6 +13,7 @@
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/string_piece.h"
 
 namespace crypto {
 
@@ -51,8 +52,7 @@ class HMAC {
   // to the constructor and the key supplied to the Init method. The HMAC is
   // returned in |digest|, which has |digest_length| bytes of storage available.
   // TODO(abarth): digest_length should be a size_t.
-  bool Sign(const std::string& data,
-            unsigned char* digest,
+  bool Sign(const base::StringPiece& data, unsigned char* digest,
             int digest_length) const;
 
   // TODO(albertb): Add a Verify method.