[MultiDeviceSetup] Make AndroidSmsPairingStateTracker observers checked.

This CL makes AndroidSmsPairingStateTracker observers checked. This will
prevent us from notifying destroyed observers.

Bug: 926429
Change-Id: I6bffbf3a6fd7eb3e1bd59a640a851a733057e229
Reviewed-on: https://chromium-review.googlesource.com/c/1449170
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#628475}(cherry picked from commit 9f1e2929993f29f35bd465be839cb98140ad0592)
Reviewed-on: https://chromium-review.googlesource.com/c/1452485
Cr-Commit-Position: refs/branch-heads/3683@{#168}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
1 file changed