Making the SHA1 entropy provider use the randomization_seed.

components/variations/entropy_provider.h

 // Copyright (c) 2012 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_VARIATIONS_ENTROPY_PROVIDER_H_
 #define COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 23 matching lines @@
   MersenneTwister mersenne_twister_;
 };
 
 // Fills |mapping| to create a bijection of values in the range of
 // [0, |mapping.size()|), permuted based on |randomization_seed|.
 void PermuteMappingUsingRandomizationSeed(uint32_t randomization_seed,
                                           std::vector<uint16_t>* mapping);
 
 }  // namespace internal
 
-// SHA1EntropyProvider is an entropy provider suitable for high entropy
-// sources. It works by taking the first 64 bits of the SHA1 hash of the
-// entropy source concatenated with the trial name and using that for the
-// final entropy value.
+// SHA1EntropyProvider is an entropy provider suitable for high entropy sources.
+// It works by taking the first 64 bits of the SHA1 hash of the entropy source
+// concatenated with the trial name, or randomization seed and using that for
+// the final entropy value.
 class SHA1EntropyProvider : public base::FieldTrial::EntropyProvider {
  public:
   // Creates a SHA1EntropyProvider with the given |entropy_source|, which
   // should contain a large amount of entropy - for example, a textual
   // representation of a persistent randomly-generated 128-bit value.
   explicit SHA1EntropyProvider(const std::string& entropy_source);
   ~SHA1EntropyProvider() override;
 
   // base::FieldTrial::EntropyProvider implementation:
   double GetEntropyForTrial(const std::string& trial_name,
                             uint32_t randomization_seed) const override;
 
  private:
   std::string entropy_source_;
 
   DISALLOW_COPY_AND_ASSIGN(SHA1EntropyProvider);
 };
 
 // PermutedEntropyProvider is an entropy provider suitable for low entropy
-// sources (below 16 bits). It uses the field trial name to generate a
-// permutation of a mapping array from an initial entropy value to a new value.
-// Note: This provider's performance is O(2^n), where n is the number of bits
-// in the entropy source.
+// sources (below 16 bits). It uses the field trial name or randomization seed
+// to generate a permutation of a mapping array from an initial entropy value to
+// a new value.  Note: This provider's performance is O(2^n), where n is the
+// number of bits in the entropy source.
 class PermutedEntropyProvider : public base::FieldTrial::EntropyProvider {
  public:
   // Creates a PermutedEntropyProvider with the given |low_entropy_source|,
   // which should have a value in the range of [0, low_entropy_source_max).
   PermutedEntropyProvider(uint16_t low_entropy_source,
                           size_t low_entropy_source_max);
   ~PermutedEntropyProvider() override;
 
   // base::FieldTrial::EntropyProvider implementation:
   double GetEntropyForTrial(const std::string& trial_name,
                             uint32_t randomization_seed) const override;
 
  protected:
   // Performs the permutation algorithm and returns the permuted value that
   // corresponds to |low_entropy_source_|.
   virtual uint16_t GetPermutedValue(uint32_t randomization_seed) const;
 
  private:
   uint16_t low_entropy_source_;
   size_t low_entropy_source_max_;
 
   DISALLOW_COPY_AND_ASSIGN(PermutedEntropyProvider);
 };
 
 }  // namespace metrics
 
 #endif  // COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_