Prepare PLUS_ADDRESS_SETTING for write support

Write support for PLUS_ADDRESS_SETTING will be necessary, since (at
least) the setting whether the user has seen a notice will be writable.

As a first step, this CL implements the bridge's GetDataForCommit(),
which was left unimplemented during the initial read-only
implementation.

Bug: 342089839
Change-Id: If6798952b59524fbd86d71e0ed3d601163701354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5672601
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1322794}