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

chrome/browser/sync/protocol/search_engine_specifics.proto

Index: chrome/browser/sync/protocol/search_engine_specifics.proto
diff --git a/chrome/browser/sync/protocol/search_engine_specifics.proto b/chrome/browser/sync/protocol/search_engine_specifics.proto
index b593b1cd3c1776abf6ef8d40727de2762158df12..effaf997d1fc3a9333462b6688ba7762b0eebf22 100644
--- a/chrome/browser/sync/protocol/search_engine_specifics.proto
+++ b/chrome/browser/sync/protocol/search_engine_specifics.proto
@@ -14,8 +14,6 @@ option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 // Properties of custom search engine sync objects.
 message SearchEngineSpecifics {
   // The description of the search engine.
@@ -61,6 +59,3 @@ message SearchEngineSpecifics {
   optional string sync_guid = 18;
 }
 
-extend EntitySpecifics {
-  optional SearchEngineSpecifics search_engine = 88610;
-}