Treat atomic_int as a trivial class member when checking for non-trivial classes.

This is a quick hack to unblock work on
https://codereview.chromium.org/2484803003. The real fix will be to check the
class after template instantiation.

BUG=663463

Review-Url: https://codereview.chromium.org/2483973003
Cr-Commit-Position: refs/heads/master@{#430773}