HQ will stay in California as long as the two refineries there are in operation. It's an advantaged market for refiners and they want to keep their refining position as long as it's profitable.
Also I think the execs still enjoy living there, so there's that.
I also believe, despite the rhetoric at times, that the California dems find the Chevron management to be much more aligned and agreeable than other O&G companies would be. California still needs their California spec gasoline for now and I doubt they would like to spin the roulette wheel on who would end up operating those assets if Chevron left. So I think it is going to continue to be a mutually agreed stalemate for the next 10-20 years.
For all intents and purposes, however, Chevron is already headquartered in Houston.