[Tab Group Sync] Handle missing synced tab group

It recently became possible for a saved tab group to not be found when
adding or updating it. Likely due to the state being different when the
posted task introduced in the below CL was run.
https://chromium-review.googlesource.com/c/chromium/src/+/5611692

In this case, rather than CHECK we should just no-op.

Fixed: 346769031
Change-Id: I944e116b441f8578b3fb66ea9916775befd847c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5626812
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1314023}