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

chrome/browser/sync/protocol/session_specifics.proto

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol datatype extension for sessions.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "sync.proto";
-
 message SessionSpecifics {
   // Unique id for the client.
   optional string session_tag = 1;
   optional SessionHeader header = 2;
   optional SessionTab tab = 3;
 }
 // Properties of session sync objects.
 message SessionHeader {
   // Each session is composed of windows.
   repeated SessionWindow window = 2;
@@ skipping 75 matching lines @@
   }
   // These qualifiers further define the transition.
   enum PageTransitionQualifier {
     CLIENT_REDIRECT = 1;
     SERVER_REDIRECT = 2;
   }
   optional PageTransition page_transition = 6 [default = TYPED];
   optional PageTransitionQualifier navigation_qualifier = 7;
 }
 
-extend EntitySpecifics {
-  optional SessionSpecifics session = 50119;
-}