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The process of stopping a datatype is asynchronous by nature, most
prominently in USS where we want to take special care about sync
metadata being cleared.

Prior to this patch, ModelTypeController could silently drop stop
events, which could lead to the DataTypeManager (effectively the UI)
think a datatype is stopped, while in reality it could remain
running. This is problematic in multiple ways, most importantly due
to the risk of leaking sync metadata or even data.

Instead, what we want is that calls to OnSyncStarting() always have a
corresponding StopSync(), and guarantee that OnSyncStarting() is not
called twice without a StopSync() call in between.

In order to do that, ModelTypeController now enters state STOPPING
if the Stop() cannot be immediately executed, because it's currently
STARTING. Once the start is finalized, stop is immediately issued and
the corresponding callback informed, which allows
ModelAssociationManager to continue its job.

Bug: 855375
Change-Id: I6e22940fcb56816c43181c34e04c4a6e53c70a21
Reviewed-on: https://chromium-review.googlesource.com/1122863
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572555}