[Sync] Convert SyncSessionSnapshot to a copy-able class.

sync/sessions/sync_session_context.h

Index: sync/sessions/sync_session_context.h
diff --git a/sync/sessions/sync_session_context.h b/sync/sessions/sync_session_context.h
index 5c217264f6c8b6ce8546dd71fdb33338f47657c2..117afc499bab1f46328fe08cf76578640ac8ea9b 100644
--- a/sync/sessions/sync_session_context.h
+++ b/sync/sessions/sync_session_context.h
@@ -27,6 +27,7 @@
 #include "base/time.h"
 #include "sync/engine/model_safe_worker.h"
 #include "sync/engine/syncer_types.h"
+#include "sync/engine/sync_engine_event.h"
 #include "sync/engine/traffic_recorder.h"
 #include "sync/sessions/debug_info_getter.h"
 
@@ -46,7 +47,6 @@ static const int kDefaultMaxCommitBatchSize = 25;
 
 namespace sessions {
 class ScopedSessionContextConflictResolver;
-struct SyncSessionSnapshot;
 class TestScopedSessionEventListener;
 
 class SyncSessionContext {
@@ -172,9 +172,6 @@ class SyncSessionContext {
   // by the user.
   ModelSafeRoutingInfo previous_session_routing_info_;
 
-  // Cache of last session snapshot information.
-  scoped_ptr<sessions::SyncSessionSnapshot> previous_session_snapshot_;
-
   // We use this to get debug info to send to the server for debugging
   // client behavior on server side.
   DebugInfoGetter* const debug_info_getter_;