Fix the hash generation algorithm to be consistent with prior implementation.

chrome/browser/prefs/pref_hash_calculator.cc

Index: chrome/browser/prefs/pref_hash_calculator.cc
diff --git a/chrome/browser/prefs/pref_hash_calculator.cc b/chrome/browser/prefs/pref_hash_calculator.cc
index 4a787a8a1abb699d8a3447c3a34caf3eee638bb8..24d771a31c45dfd65bb2850b4aa2a5415574b1f7 100644
--- a/chrome/browser/prefs/pref_hash_calculator.cc
+++ b/chrome/browser/prefs/pref_hash_calculator.cc
@@ -16,6 +16,17 @@
 
 namespace {
 
+// Calculates an HMAC of |message| using |key|, encoded as a hexadecimal string.
+std::string GetHMACHex(const std::string& key, const std::string& message) {
+  crypto::HMAC hmac(crypto::HMAC::SHA256);
+  std::vector<uint8> digest(hmac.DigestLength());
+  if (!hmac.Init(key) || !hmac.Sign(message, &digest[0], digest.size())) {
+    NOTREACHED();
+    return std::string();
+  }
+  return base::HexEncode(digest.data(), digest.size());
+}
+
 // Renders |value| as a string. |value| may be NULL, in which case the result
 // is an empty string.
 std::string ValueAsString(const base::Value* value) {
@@ -42,25 +53,28 @@ std::string CalculateFromValueAndComponents(
     const std::string& seed,
     const base::Value* value,
     const std::vector<std::string>& extra_components) {
-  static const size_t kSHA256DigestSize = 32;
-
   std::string message = JoinString(extra_components, "") + ValueAsString(value);
+  return GetHMACHex(seed, message);
+}
 
-  crypto::HMAC hmac(crypto::HMAC::SHA256);
-  unsigned char digest[kSHA256DigestSize];
-  if (!hmac.Init(seed) || !hmac.Sign(message, digest, arraysize(digest))) {
-    NOTREACHED();
-    return std::string();
-  }
 
-  return base::HexEncode(digest, arraysize(digest));
+// Generates a device ID based on the input device ID. The derived device ID has
+// no useful properties beyond those of the input device ID except that it is
+// consistent with previous implementations.
+std::string GenerateDeviceIdLikePrefMetricsServiceDid(
+    const std::string& original_device_id) {
+  if (original_device_id.empty())
+    return std::string();
+  return StringToLowerASCII(
+      GetHMACHex(original_device_id, "PrefMetricsService"));
 }
 
 }  // namespace
 
 PrefHashCalculator::PrefHashCalculator(const std::string& seed,
                                        const std::string& device_id)
-    : seed_(seed), device_id_(device_id) {}
+    : seed_(seed),
+      device_id_(GenerateDeviceIdLikePrefMetricsServiceDid(device_id)) {}
 
 std::string PrefHashCalculator::Calculate(const std::string& path,
                                           const base::Value* value) const {