Avoid some string copies in RAPPOR code.

components/rappor/byte_vector_utils.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
 #define COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <vector>
 
 #include "base/macros.h"
+#include "base/strings/string_piece.h"
 #include "components/rappor/rappor_parameters.h"
 #include "crypto/hmac.h"
 
 namespace rappor {
 
 // A vector of 8-bit integers used to store a set of binary bits.
 typedef std::vector<uint8_t> ByteVector;
 
 // Converts the lowest |size| bytes of |value| into a ByteVector.
 void Uint64ToByteVector(uint64_t value, size_t size, ByteVector* output);
@@ skipping 52 matching lines @@
 // we're choosing to use HMAC_DRBG here, as it is one of the best studied
 // and standardized ways of generating deterministic, unpredictable sequences
 // based on a secret seed.
 class HmacByteVectorGenerator : public ByteVectorGenerator {
  public:
   // Constructor takes the size of the vector to generate, along with a
   // |entropy_input| and |personalization_string| to seed the pseudo-random
   // number generator.  The string parameters are treated as byte arrays.
   HmacByteVectorGenerator(size_t byte_count,
                           const std::string& entropy_input,
-                          const std::string& personalization_string);
+                          base::StringPiece personalization_string);
 
   ~HmacByteVectorGenerator() override;
 
   // Generates a random string suitable for passing to the constructor as
   // |entropy_input|.
   static std::string GenerateEntropyInput();
 
   // Key size required for 128-bit security strength (including nonce).
   static const size_t kEntropyInputSize;
 
@@ skipping 14 matching lines @@
 
   // Total number of bytes streamed from the HMAC_DRBG Generate Process.
   size_t generated_bytes_;
 
   DISALLOW_ASSIGN(HmacByteVectorGenerator);
 };
 
 }  // namespace rappor
 
 #endif  // COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_