[Sync] Wrap model type specifics in oneof.

components/sync/protocol/sync.proto

Index: components/sync/protocol/sync.proto
diff --git a/components/sync/protocol/sync.proto b/components/sync/protocol/sync.proto
index a9301d36bb91edf756f1da6551ed561e4b7b6342..b5e29414d12daf8439fc41c5ec64a4e88b000365 100644
--- a/components/sync/protocol/sync.proto
+++ b/components/sync/protocol/sync.proto
@@ -102,44 +102,44 @@ message EntitySpecifics {
   // noise in logs and debugging contexts, and creating a divergent subset of
   // tags would only make things a bit more confusing.
 
-  optional AutofillSpecifics autofill = 31729;
-  optional BookmarkSpecifics bookmark = 32904;
-  optional PreferenceSpecifics preference = 37702;
-  optional TypedUrlSpecifics typed_url = 40781;
-  optional ThemeSpecifics theme = 41210;
-  optional AppNotification app_notification = 45184;
-  optional PasswordSpecifics password = 45873;
-  optional NigoriSpecifics nigori = 47745;
-  optional ExtensionSpecifics extension = 48119;
-  optional AppSpecifics app = 48364;
-  optional SessionSpecifics session = 50119;
-  optional AutofillProfileSpecifics autofill_profile = 63951;
-  optional SearchEngineSpecifics search_engine = 88610;
-  optional ExtensionSettingSpecifics extension_setting = 96159;
-  optional AppSettingSpecifics app_setting = 103656;
-  optional HistoryDeleteDirectiveSpecifics history_delete_directive = 150251;
-  optional SyncedNotificationSpecifics synced_notification = 153108;
-  optional SyncedNotificationAppInfoSpecifics synced_notification_app_info =
-      235816;
-  optional DeviceInfoSpecifics device_info = 154522;
-  optional ExperimentsSpecifics experiments = 161496;
-  optional PriorityPreferenceSpecifics priority_preference = 163425;
-  optional DictionarySpecifics dictionary = 170540;
-  optional FaviconTrackingSpecifics favicon_tracking = 181534;
-  optional FaviconImageSpecifics favicon_image = 182019;
-  optional ManagedUserSettingSpecifics managed_user_setting = 186662;
-  optional ManagedUserSpecifics managed_user = 194582;
-  optional ManagedUserSharedSettingSpecifics managed_user_shared_setting =
-      202026;
-  optional ManagedUserWhitelistSpecifics managed_user_whitelist = 306060;
-  optional ArticleSpecifics article = 223759;
-  optional AppListSpecifics app_list = 229170;
-  optional WifiCredentialSpecifics wifi_credential = 218175;
-  optional AutofillWalletSpecifics autofill_wallet = 306270;
-  optional WalletMetadataSpecifics wallet_metadata = 330441;
-  optional ArcPackageSpecifics arc_package = 340906;
-  optional PrinterSpecifics printer = 410745;
-  optional ReadingListSpecifics reading_list = 411028;
+  oneof specifics_variant {
+    AutofillSpecifics autofill = 31729;
+    BookmarkSpecifics bookmark = 32904;
+    PreferenceSpecifics preference = 37702;
+    TypedUrlSpecifics typed_url = 40781;
+    ThemeSpecifics theme = 41210;
+    AppNotification app_notification = 45184;
+    PasswordSpecifics password = 45873;
+    NigoriSpecifics nigori = 47745;
+    ExtensionSpecifics extension = 48119;
+    AppSpecifics app = 48364;
+    SessionSpecifics session = 50119;
+    AutofillProfileSpecifics autofill_profile = 63951;
+    SearchEngineSpecifics search_engine = 88610;
+    ExtensionSettingSpecifics extension_setting = 96159;
+    AppSettingSpecifics app_setting = 103656;
+    HistoryDeleteDirectiveSpecifics history_delete_directive = 150251;
+    SyncedNotificationSpecifics synced_notification = 153108;
+    SyncedNotificationAppInfoSpecifics synced_notification_app_info = 235816;
+    DeviceInfoSpecifics device_info = 154522;
+    ExperimentsSpecifics experiments = 161496;
+    PriorityPreferenceSpecifics priority_preference = 163425;
+    DictionarySpecifics dictionary = 170540;
+    FaviconTrackingSpecifics favicon_tracking = 181534;
+    FaviconImageSpecifics favicon_image = 182019;
+    ManagedUserSettingSpecifics managed_user_setting = 186662;
+    ManagedUserSpecifics managed_user = 194582;
+    ManagedUserSharedSettingSpecifics managed_user_shared_setting = 202026;
+    ManagedUserWhitelistSpecifics managed_user_whitelist = 306060;
+    ArticleSpecifics article = 223759;
+    AppListSpecifics app_list = 229170;
+    WifiCredentialSpecifics wifi_credential = 218175;
+    AutofillWalletSpecifics autofill_wallet = 306270;
+    WalletMetadataSpecifics wallet_metadata = 330441;
+    ArcPackageSpecifics arc_package = 340906;
+    PrinterSpecifics printer = 410745;
+    ReadingListSpecifics reading_list = 411028;
+  }
 }
 
 message SyncEntity {