[Sync] Add plumbing of context from client to server

sync/engine/get_updates_processor_unittest.cc

Index: sync/engine/get_updates_processor_unittest.cc
diff --git a/sync/engine/get_updates_processor_unittest.cc b/sync/engine/get_updates_processor_unittest.cc
index b8ea889dbc4ffb76c416b00e045f54768e8edbe4..7c87fed098634e955f9e8c4e62baaa90b74ef6c2 100644
--- a/sync/engine/get_updates_processor_unittest.cc
+++ b/sync/engine/get_updates_processor_unittest.cc
@@ -53,6 +53,10 @@ class GetUpdatesProcessorTest : public ::testing::Test {
           response->add_new_progress_marker();
       marker->set_data_type_id(GetSpecificsFieldNumberFromModelType(it.Get()));
       marker->set_token("foobarbaz");
+      sync_pb::DataTypeContext* context = response->add_context_mutations();
+      context->set_data_type_id(GetSpecificsFieldNumberFromModelType(it.Get()));
+      context->set_version(1);
+      context->set_context("context");
     }
 
     response->set_changes_remaining(0);