Michael Redmond mentions this in the AlphaGo vs AlphaGo review series he's doing with the AGA. AlphaGo selfplay games are with 7.5 komi under Chinese rules, and apparently, Deepmind has stated that black vs white wins is almost exactly 50/50. IIRC Redmond mentioned that white (?) only had some sub 1% advantage in the entire self-play corpus.