GitLab

Expose Registry as read-only collection.

Users can write:
  for (auto& kv : registrar->registry()) {
    const WebApp* web_app = kv.second.get();
    ...
  }

Add IsEmpty and UnregisterAll methods.

Bug: 891172
Change-Id: I17ad8a5b569eeffb614adeec5575dc6804ce5251
Reviewed-on: https://chromium-review.googlesource.com/c/1295611
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601855}