Some code in //content (e.g. CrossSiteDocumentResourceHandler::LogBlockedResponseOnUIThread after https://crrev.com/c/833021) logs UKM events.  Such code should have corresponding //content-level tests.  Writing such tests is difficult today, because UKM test utilities live in //components (which //content cannot depend on in general).

Let's use this bug to 1) track adding new tests and 2) moving UKM test utils from //components to //services (if possible).