Allow inherited constructors and add a sample usage.

This was OKed on the discussion thread back in July, but never added.

BUG=none
TEST=none

Review-Url: https://codereview.chromium.org/2468633003
Cr-Original-Commit-Position: refs/heads/master@{#429716}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3e35c53fdc3e68aa70efbca7f49a8494df79db1b