Good call on the big google in the sky. If it was a veledrome, someone flying over looking for pools to skate would have seen it and since skateboarding and motorcycles go hand in hand these days it would be a Hollywood hipster mecca.
Well, guaranteed the owners of the house bought it for some reason... Perhaps a knock on the door with a reasonable excuse might get you a closer look (and a go-around?), I'd be pestering those people for a race around the track, 4-shore!
It is a cistern, build by the Department of Water and Power, for the L.A. Fire department's snorkel helicopters to use for fighting brush fires in the area, rather that to fly to the ocean. It doesn't appear to have been finished