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SyncableServices don't actually consume this field, the only caller
to GetId() being GenericChangeProcessor. Changing the API towards
SyncableServices would require a huge refactoring, so let's instead
allow that SyncData may not be populated, and DCHECK-fail lazily on
read (in case a SyncableService implementation is changed in the
future to consume this field).

The rationale behind is that future patches will integrate
SyncableService instantes within the USS framework, where the ID
doesn't have any semantics.

As implementation detail: because local vs remote was previously
implemented verifying id_ != kInvalidId, we need to instead
introduce a dedicated bool member.

Bug: 870624
Change-Id: I75aa92e69a861ea6af58327a14406bc086233043
Reviewed-on: https://chromium-review.googlesource.com/1205393
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594646}