[sync] Avoid START_DEFERRED if not actually deferred

The specific behavioral change affects a short-lived transition
between SyncServiceImpl::TryStart() and
SyncServiceImpl::TryStartImpl(), where the first posts a tasks to
run the second.

During this short interval, the transport is not really deferred
and using INITIALIZING appears more appealing.

In practice, most calling sites do not distinguish between
START_DEFERRED and INITIALIZING.

Change-Id: I7285e0a6b3aea38efdf824660655e0788c4308c9
Bug: 40946404
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5458155
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289870}