based on your two suggestions, I would go w/the latter: it doesn't quite look like bittercress, especially in the flower, so I don't think that's it.
that said: this doesn't grow in Southern California, so I've never seen it...but from google image search, I think Cochlearia genus might be a good fit...especially when you consider this picture and compare the flower to yours: http://www.botanical.com/botanical/mgmh/s/scurvy35.html
My thoughts also, thanks for your input..... I had this suggested to me earlier this evening so I'm going to look into it further to see if it's edible, thanks.