Use {To/From}JavaTime for policy timestamps

components/policy/proto/device_management_local.proto

Index: components/policy/proto/device_management_local.proto
diff --git a/components/policy/proto/device_management_local.proto b/components/policy/proto/device_management_local.proto
index 271c7bf3d90cde04743bfad916000af89dcd65f3..a85268fb12128f816e2b3353862f845fc391df8d 100644
--- a/components/policy/proto/device_management_local.proto
+++ b/components/policy/proto/device_management_local.proto
@@ -15,8 +15,9 @@ message CachedCloudPolicyResponse {
   // The DevicePolicyResponse wrapped by this message.
   optional DevicePolicyResponse device_policy = 1;
   // Timestamp noting when the |unmanaged| flag was set. The meaning is
-  // milliseconds since 1970. When caching (deprecated) DevicePolicyResponses,
-  // this timestamp also notes when the response was cached.
+  // milliseconds since Epoch in UTC timezone (Java time). When caching
+  // (deprecated) DevicePolicyResponses, this timestamp also notes when the
+  // response was cached.
   optional uint64 timestamp = 2;
   // Flag that is set to true if this device is not managed.
   optional bool unmanaged = 3;