[Sync::USS] Don't populate client tags for bookmark tombstones

Before this CL:
NonBlockingTypeCommitContribution used to populate the client
defined unique tag for bookmark tombstones with an empty string. This
is because it used the check if specifics has bookmarks to identify
the bookmark commits. However, for tombstones, specifics are empty and
hence the check fails.

After this CL:
The model type is plumbed through and checked explicitly instead of
checking the specifics.

In addition, this CL also makes sure the client tag isn't populated for
Nigori data type.

Bug: 963895
Change-Id: I2c04e758fc341f92cd3ffe46b38579b5d98fead6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615202
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660389}
3 files changed