Fix base::StringPiece calling conventions in //crypto.

Per the documentation in the header, base::StringPiece should be passed
by value, not by const reference.

Bug: 
Change-Id: I21351fcb901f5ef89fa7b84af0721d483b333810
Reviewed-on: https://chromium-review.googlesource.com/754584
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514181}

diff --git a/crypto/aead.h b/crypto/aead.h
index 220ae420..7c64eaa9 100644
--- a/crypto/aead.h
+++ b/crypto/aead.h

@@ -27,14 +27,14 @@
 
   void Init(const std::string* key);
 
-  bool Seal(const base::StringPiece& plaintext,
-            const base::StringPiece& nonce,
-            const base::StringPiece& additional_data,
+  bool Seal(base::StringPiece plaintext,
+            base::StringPiece nonce,
+            base::StringPiece additional_data,
             std::string* ciphertext) const;
 
-  bool Open(const base::StringPiece& ciphertext,
-            const base::StringPiece& nonce,
-            const base::StringPiece& additional_data,
+  bool Open(base::StringPiece ciphertext,
+            base::StringPiece nonce,
+            base::StringPiece additional_data,
             std::string* plaintext) const;
 
   size_t KeyLength() const;