The test started flaking with this build: https://build.chromium.org/p/chromium.gpu/builders/Mac%20Retina%20Debug%20%28AMD%29/builds/6972

But the only change there is the revert for a toolchain change that broke the bots. It is an assertion failure on the pixel output, here is a sample output: http://chromium-browser-gpu-tests.commondatastorage.googleapis.com/view_test_results.html?0db10a4d79a10a105cb06feae8c025df8e477f43_Mac_Retina_Debug_AMD__telemetry

xlai@, could you take a look and verify if a change looks like a possible culprit to you? It might help to take a look at a few builds before the first failing build on the bot (https://build.chromium.org/p/chromium.gpu/builders/Mac%20Retina%20Debug%20%28AMD%29). The failure started with build #6972.