sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/protocol/sync.proto

Index: chrome/browser/sync/protocol/sync.proto
diff --git a/chrome/browser/sync/protocol/sync.proto b/chrome/browser/sync/protocol/sync.proto
index 41ef93302e99faec688e4717b9d228857c8e43f5..b1236ba60c9b9f6b6b11e8b7b8c0b7c6b482f4ab 100644
--- a/chrome/browser/sync/protocol/sync.proto
+++ b/chrome/browser/sync/protocol/sync.proto
@@ -14,6 +14,20 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
+import "app_notification_specifics.proto";
+import "app_setting_specifics.proto";
+import "app_specifics.proto";
+import "autofill_specifics.proto";
+import "bookmark_specifics.proto";
+import "extension_setting_specifics.proto";
+import "extension_specifics.proto";
+import "nigori_specifics.proto";
+import "password_specifics.proto";
+import "preference_specifics.proto";
+import "search_engine_specifics.proto";
+import "session_specifics.proto";
+import "theme_specifics.proto";
+import "typed_url_specifics.proto";
 import "encryption.proto";
 import "sync_enums.proto";
 import "client_commands.proto";
@@ -60,7 +74,22 @@ message EntitySpecifics {
   // EntitySpecifics when it contains unrecognized extensions.  In this
   // way, it is possible to add new datatype fields without having
   // to update the server.
-  extensions 30000 to max;
+
+  optional AutofillSpecifics autofill = 31729;
+  optional ExtensionSettingSpecifics extension_setting = 96159;
+  optional ExtensionSpecifics extension = 48119;
+  optional NigoriSpecifics nigori = 47745;
+  optional PasswordSpecifics password = 45873;
+  optional PreferenceSpecifics preference = 37702;
+  optional SearchEngineSpecifics search_engine = 88610;
+  optional SessionSpecifics session = 50119;
+  optional ThemeSpecifics theme = 41210;
+  optional TypedUrlSpecifics typed_url = 40781;
+  optional BookmarkSpecifics bookmark = 32904;
+  optional AppNotification app_notification = 45184;
+  optional AppSettingSpecifics app_setting = 103656;
+  optional AppSpecifics app = 48364;
+  optional AutofillProfileSpecifics autofill_profile = 63951;
 }
 
 message SyncEntity {