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

chrome/browser/sync/protocol/preference_specifics.proto

Index: chrome/browser/sync/protocol/preference_specifics.proto
diff --git a/chrome/browser/sync/protocol/preference_specifics.proto b/chrome/browser/sync/protocol/preference_specifics.proto
index 72bc3750f1270e2ff25897568928a5797f6dfc8c..bf98afa73843ab735cf5e2784156c4ecf8ded6af 100644
--- a/chrome/browser/sync/protocol/preference_specifics.proto
+++ b/chrome/browser/sync/protocol/preference_specifics.proto
@@ -14,14 +14,9 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 // Properties of preference sync objects.
 message PreferenceSpecifics {
   optional string name = 1;
   optional string value = 2;
 }
 
-extend EntitySpecifics {
-  optional PreferenceSpecifics preference = 37702;
-}