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

sync/protocol/session_specifics.proto

Index: sync/protocol/session_specifics.proto
diff --git a/sync/protocol/session_specifics.proto b/sync/protocol/session_specifics.proto
index e2b441d86149106b125481b5ebf5c92d2b6f483d..43892435ccbb8b3067979cf841897835e5cba8cf 100644
--- a/sync/protocol/session_specifics.proto
+++ b/sync/protocol/session_specifics.proto
@@ -91,7 +91,7 @@ message SessionTab {
 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;
@@ -126,5 +126,10 @@ message TabNavigation {
   }
   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;
 }