Only do aggressive metro viewer termination in tests

ash/test/test_metro_viewer_process_host.cc

Index: ash/test/test_metro_viewer_process_host.cc
diff --git a/ash/test/test_metro_viewer_process_host.cc b/ash/test/test_metro_viewer_process_host.cc
index 85df3e21cbccfb6f64faad4fb6aaa9ea2dbdf6f5..91b8795ce43919002ba7e08c296d2e7c297b07b6 100644
--- a/ash/test/test_metro_viewer_process_host.cc
+++ b/ash/test/test_metro_viewer_process_host.cc
@@ -22,6 +22,10 @@ TestMetroViewerProcessHost::TestMetroViewerProcessHost(
 TestMetroViewerProcessHost::~TestMetroViewerProcessHost() {
 }
 
+void TestMetroViewerProcessHost::TerminateViewer() {
+  DisposeViewer(GetViewerProcessId(), true);
+}
+
 void TestMetroViewerProcessHost::OnChannelError() {
   closed_unexpectedly_ = true;
   aura::RemoteWindowTreeHostWin::Instance()->Disconnected();