Use {To/From}JavaTime for policy timestamps

components/policy/core/common/cloud/cloud_policy_validator.cc

Index: components/policy/core/common/cloud/cloud_policy_validator.cc
diff --git a/components/policy/core/common/cloud/cloud_policy_validator.cc b/components/policy/core/common/cloud/cloud_policy_validator.cc
index bcd24b23f834e397ad486c0e6fb789d0de995673..8a51556ed1b9282de9260926bdd0f5d957a2f821 100644
--- a/components/policy/core/common/cloud/cloud_policy_validator.cc
+++ b/components/policy/core/common/cloud/cloud_policy_validator.cc
@@ -53,8 +53,7 @@ void CloudPolicyValidatorBase::ValidateTimestamp(
     base::Time not_before,
     ValidateTimestampOption timestamp_option) {
   validation_flags_ |= VALIDATE_TIMESTAMP;
-  timestamp_not_before_ =
-      (not_before - base::Time::UnixEpoch()).InMilliseconds();
+  timestamp_not_before_ = not_before.ToJavaTime();
   timestamp_option_ = timestamp_option;
 }
 
@@ -146,9 +145,7 @@ void CloudPolicyValidatorBase::ValidateAgainstCurrentPolicy(
   std::string expected_dm_token;
   std::string expected_device_id;
   if (policy_data) {
-    last_policy_timestamp =
-        base::Time::UnixEpoch() +
-        base::TimeDelta::FromMilliseconds(policy_data->timestamp());
+    last_policy_timestamp = base::Time::FromJavaTime(policy_data->timestamp());
     expected_dm_token = policy_data->request_token();
     expected_device_id = policy_data->device_id();
   }