[Sync::USS] Fix sending unnecessary bookmarks tombstones

Before this CL: the processor will try to commit tombstones for
entities that has never been committed.

After this CL: if the processor is sure that the server never heard
or an entity, it will never a tombstone for it.

Bug: 963365
Change-Id: Iae101094b80eb2c4d080cb92b253a2886373aedf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609851
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659993}
6 files changed