[Sync] Add plumbing of context from client to server

sync/engine/get_updates_processor.cc

Index: sync/engine/get_updates_processor.cc
diff --git a/sync/engine/get_updates_processor.cc b/sync/engine/get_updates_processor.cc
index eb1d7be2e82987f9cff188b52a485e0f260421d7..d94f12cfa82ab5858e5bf444e92cd2419031e730 100644
--- a/sync/engine/get_updates_processor.cc
+++ b/sync/engine/get_updates_processor.cc
@@ -178,7 +178,13 @@ void GetUpdatesProcessor::PrepareGetUpdates(
         get_updates->add_from_progress_marker();
     handler_it->second->GetDownloadProgress(progress_marker);
     progress_marker->clear_gc_directive();
+
+    sync_pb::DataTypeContext context;
+    handler_it->second->GetDataTypeContext(&context);
+    if (!context.context().empty())
+      get_updates->add_client_contexts()->Swap(&context);
   }
+
   delegate_.HelpPopulateGuMessage(get_updates);
 }
 
@@ -314,6 +320,28 @@ bool GetUpdatesProcessor::ProcessGetUpdatesResponse(
   DCHECK(progress_marker_iter == progress_index_by_type.end() &&
          updates_iter == updates_by_type.end());
 
+  for (int i = 0; i < gu_response.context_mutations_size(); ++i) {
+    const sync_pb::DataTypeContext& context = gu_response.context_mutations(i);
+    syncer::ModelType type =
+        GetModelTypeFromSpecificsFieldNumber(context.type());
+    if (!syncer::IsRealDataType(type)) {
+      DLOG(WARNING) << "Received datatype context mutation with invalid type.";
+      continue;
+    }
+
+    UpdateHandlerMap::iterator update_handler_iter =
+        update_handler_map_->find(type);
+
+    if (update_handler_iter != update_handler_map_->end()) {
+      update_handler_iter->second->ProcessDataTypeContextMutation(context);
+    } else {
+      DLOG(WARNING)
+          << "Ignoring received context mutations of a type we can't handle.  "
+          << "Type is: " << ModelTypeToString(type);
+      continue;
+    }
+  }
+
   return true;
 }