[Sync] Add per-navigation timestamps/unique ids to tab sync.

sync/protocol/session_specifics.proto

 // Copyright (c) 2012 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";
@@ skipping 73 matching lines @@
     TYPE_WEB_FAVICON = 1;
   }
   optional FaviconType favicon_type = 9;
   // The url of the actual favicon (as opposed to the page using the favicon).
   optional string favicon_source = 11;
 }
 
 message TabNavigation {
   // The index in the NavigationController. If this is -1, it means this
   // TabNavigation is bogus.
-  optional int32 index = 1 [default = -1];
+  // optional int32 index = 1 [default = -1];  // obsolete.
   // The virtual URL, when nonempty, will override the actual URL of the page
   // when we display it to the user.
   optional string virtual_url = 2;
   // The referring URL, which can be empty.
   optional string referrer = 3;
   // The title of the page.
   optional string title = 4;
   // Content state is an opaque blob created by WebKit that represents the
   // state of the page. This includes form entries and scroll position for each
   // frame.
@@ skipping 14 matching lines @@
     CHAIN_START = 12;
     CHAIN_END = 13;
   }
   // 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;
+  // The unique navigation id (within this client).
+  optional int32 unique_id = 8;
+  // Timestamp for when this navigation last occurred (in client time).
+  // If the user goes back/foward in history the timestamp may refresh.
+  optional int64 timestamp = 9;
 }