[Sync] Deprecate sync session page state field

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 2a0d9031bef34828259cfcff65a5ca221a035747..47a204bef5bb6228c96ad53d8d05263ea8a8f9ad 100644
--- a/sync/protocol/session_specifics.proto
+++ b/sync/protocol/session_specifics.proto
@@ -99,7 +99,7 @@ message TabNavigation {
   // 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.
-  optional string state = 5;
+  // optional string state = 5;  // obsolete.

[maxbogue, 2015/10/23 00:53:28]

Is it customary to comment out deprecated fields like this in Chromium? I've
only ever seen it done by adding [deprecated = true] elsewhere. Or removing the
field entirely if the proto is never stored, which I don't think applies here.

   // The core transition type.
   optional SyncEnums.PageTransition page_transition = 6 [default = TYPED];
   // If this transition was triggered by a redirect, the redirect type.