GitLab

This will eventually take the place of
GattClientManager::GetNumConnected. This moves bluetooth closer to being
fully async

Bug: 76155468
Test: chromecast/device/bluetooth/le/gatt_client_manager_impl_test.cc
Change-Id: I53d7eb0b2d5743165c1df5fe64903b86012a0346
Reviewed-on: https://chromium-review.googlesource.com/1147879Reviewed-by: Bailey Forrest <bcf@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Eli Ribble <eliribble@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577748}