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

chrome/browser/sync/protocol/typed_url_specifics.proto

Index: chrome/browser/sync/protocol/typed_url_specifics.proto
diff --git a/chrome/browser/sync/protocol/typed_url_specifics.proto b/chrome/browser/sync/protocol/typed_url_specifics.proto
index 4bc93e301a76543ed3abdf1a4a0b781e19d2dee5..42a66a9f73040c29c7356779e9e5184de140d594 100644
--- a/chrome/browser/sync/protocol/typed_url_specifics.proto
+++ b/chrome/browser/sync/protocol/typed_url_specifics.proto
@@ -14,8 +14,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 // Properties of typed_url sync objects - fields correspond to similarly named
 // fields in history::URLRow.
 message TypedUrlSpecifics {
@@ -41,7 +39,3 @@ message TypedUrlSpecifics {
   // arrays must be the same length.
   repeated int32 visit_transitions = 8 [packed=true];
 }
-
-extend EntitySpecifics {
-  optional TypedUrlSpecifics typed_url = 40781;
-}