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Fixes a flaky issue introduced in the recent tabs submenu tests due to
inspecting possibly unset timestamp values. Also fixes a pre-existing issue
where sync ids for tabs were not being set, which the refactor brought
to light.

BUG=671902, 671902
TBR=msw

Review-Url: https://codereview.chromium.org/2562853002
Cr-Commit-Position: refs/heads/master@{#437604}