Implement SyncableServiceBasedBridge::IsEntityDataValid

This change allows to do less validation in bridges while merging remote
changes as sync should filter invalid entries before passing them to
bridges.

The implementation always trivially returns true as this this bridge is
meant to cache locally a copy of the server-side data in proto format
as-is.

Bug: 40677711
Change-Id: Ic2b01bbc5630dc0c75ae73654f31b8d6afd0e818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578369
Reviewed-by: Jood Hajeer <jood@google.com>
Commit-Queue: Mikalai Parafeniuk <mipa@google.com>
Reviewed-by: Mahmoud Rashad <mmrashad@google.com>
Cr-Commit-Position: refs/heads/main@{#1464157}