sendMessage: in preview should populate history and return a chat response failed

Note: #484 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky. ---- commit: e9abf251d4802ccb6f5a72770baf5dd294c31cfd buildURL: [Build Status](https://source.cloud.google.com/results/invocations/37cb0624-ea9e-4b95-9afd-36a337a0efe9), [Sponge](http://sponge2/37cb0624-ea9e-4b95-9afd-36a337a0efe9) status: failed