Update the remaining include paths of base/string_piece.h to its new location.

string_piece.h was moved into base/strings/ in r191206 -
https://chromiumcodereview.appspot.com/12982018/

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Review URL: https://codereview.chromium.org/14272007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194693 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/crypto/hmac.h b/crypto/hmac.h
index d527d16..c7b22fa 100644
--- a/crypto/hmac.h
+++ b/crypto/hmac.h
@@ -11,7 +11,7 @@
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "crypto/crypto_export.h"
 
 namespace crypto {