This is a follow up to the comments on https://codereview.chromium.org/2897563002/ . Reviewers ask for unit tests and renjieliu@ points out that this is currently very difficult because the translate code is forced to depend directly on the concrete class's definition, which is currently called UserEventService.

We should be able to facilitate unit tests by simply breaking the interface and implementation into two separate classes, and providing a test implementation that, instead of calling into a bridge, simply accumulates changes for later verification against.

It's my understanding that the existing KeyedServiceFactory::SetTestingFactory() should be sufficient once the above split is created.