[Sync] Adding missing DependsOn to *ProfileSyncServiceFactory.

docs/sync/model_api.md

 # Chrome Sync's Model API
 
 Chrome Sync operates on discrete, explicitly defined model types (bookmarks,
 preferences, tabs, etc). These model types are individually responsible for
 implementing their own local storage and responding to remote changes. This
 guide is for developers interested in syncing data for their model type to the
 cloud using Chrome Sync. It describes the newest version of the API, known as
 Unified Sync and Storage (USS). There is also the deprecated [SyncableService
 API] (aka Directory), which as of early 2016 is still used by most model types.
 
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 *   Define your specifics proto in [`//components/sync/protocol/`][protocol].
 *   Add a field for it to [`EntitySpecifics`][EntitySpecifics].
 *   Add it to the [`ModelType`][ModelType] enum and
     [`kModelTypeInfoMap`][info_map].
 *   Add it to the [proto value conversions][conversions] files.
 *   Register a [`ModelTypeController`][ModelTypeController] for your type in
     [`ProfileSyncComponentsFactoryImpl::RegisterDataTypes`][RegisterDataTypes].
 *   Tell sync how to access your `ModelTypeSyncBridge` in
     [`ChromeSyncClient::GetSyncBridgeForModelType`][GetSyncBridge].
+*   Add your KeyedService dependency to
+    [`ProfileSyncServiceFactory`][ProfileSyncServiceFactory].
 *   Add to the [start order list][kStartOrder].
 *   Add an field for encrypted data to [`NigoriSpecifics`][NigoriSpecifics].
 *   Add to two encrypted types translation functions in
     [`nigori_util.cc`][nigori_util].
 *   Add a [preference][pref_names] for tracking whether your type is enabled.
 *   Map your type to the pref in [`GetPrefNameForDataType`][GetPrefName].
 *   Check whether you should be part of a [pref group][RegisterPrefGroup].
 *   Add to the `SyncModelTypes` enum and `SyncModelType` suffix in
     [`histograms.xml`][histograms].
 *   Add to the [`SYNC_DATA_TYPE_HISTOGRAM`][DataTypeHistogram] macro.
 
 [protocol]: https://cs.chromium.org/chromium/src/components/sync/protocol/
 [ModelType]: https://cs.chromium.org/chromium/src/components/sync/base/model_type.h
 [info_map]: https://cs.chromium.org/search/?q="kModelTypeInfoMap%5B%5D"+file:model_type.cc
 [conversions]: https://cs.chromium.org/chromium/src/components/sync/protocol/proto_value_conversions.h
 [ModelTypeController]: https://cs.chromium.org/chromium/src/components/sync/driver/model_type_controller.h
 [RegisterDataTypes]: https://cs.chromium.org/search/?q="ProfileSyncComponentsFactoryImpl::RegisterDataTypes"
 [GetSyncBridge]: https://cs.chromium.org/search/?q=GetSyncBridgeForModelType+file:chrome_sync_client.cc
+[ProfileSyncServiceFactory]: https://cs.chromium.org/search/?q=:ProfileSyncServiceFactory%5C(%5C)
 [kStartOrder]: https://cs.chromium.org/search/?q="kStartOrder[]"
 [NigoriSpecifics]: https://cs.chromium.org/chromium/src/components/sync/protocol/nigori_specifics.proto
 [nigori_util]: https://cs.chromium.org/chromium/src/components/sync/syncable/nigori_util.cc
 [pref_names]: https://cs.chromium.org/chromium/src/components/sync/base/pref_names.h
 [GetPrefName]: https://cs.chromium.org/search/?q=::GetPrefNameForDataType+file:sync_prefs.cc
 [RegisterPrefGroup]: https://cs.chromium.org/search/?q=::RegisterPrefGroups+file:sync_prefs.cc
 [histograms]: https://cs.chromium.org/chromium/src/tools/metrics/histograms/histograms.xml
 [DataTypeHistogram]: https://cs.chromium.org/chromium/src/components/sync/base/data_type_histogram.h
 
 ## Testing
 
 The [`TwoClientUssSyncTest`][UssTest] suite is probably a good place to start
 for integration testing. Especially note the use of a `StatusChangeChecker` to
 wait for events to happen.
 
 [UssTest]: https://cs.chromium.org/chromium/src/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc