[AF Wallet] Notify individual changes from the wallet data bridge.

This CL refines the way the wallet data bridge notifies about changes it
writes to the local database. This CL required a minor extension in the
AutofillWebDataBackend interface.

The changes are not guarded behind a feature flag for wallet_metadata
uss migration as they are backwards compatible for all current
implementations of AutofillWebDataServiceObserverOnDBSequence.

Bug: 853688
Change-Id: I3f9bd3e48c4591dddf3f645edef63d51cc8fbff8
Reviewed-on: https://chromium-review.googlesource.com/1236693
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593184}
12 files changed