[Trusted Types] Add user counter for empty policy names.

The use counter is meant to gather data for standardization decision.

See: https://github.com/w3c/trusted-types/issues/466#issuecomment-1988995332
Change-Id: Ib490f5535a804e3fa69b1734dd8fb3481cfc4486
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5366584


Reviewed-by: Yifan Luo <lyf@chromium.org>
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1272679}