Add UseCounter for unknown namespace usage in selectors.

R=chrishtr@chromium.org

Bug: 901938
Change-Id: I9e20fe19f2c97b98f8250c6c9914512b5a73d8c7
Reviewed-on: https://chromium-review.googlesource.com/c/1318891
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606845}
4 files changed