Gittes concludes that they're Noah Cross's. From the script:
GITTES: An obituary column... can you read in this light?
CROSS: Yes... I think I can manage...
Cross dips into his coat pocket and pulls out a pair of rimless glasses.. He puts them on, reads. Gittes stares at the bifocal lenses as Cross continues to look through the obituary column. He looks up.
CROSS: What does this mean?
GITTES: -- that you killed Hollis Mulwray --
Gittes is holding the bifocals with the broken lens now.
GITTES: (continuing) -- right here, in this pond. You drowned him... and you left these.
And since Noah takes them away from him as to not incriminate himself, Gittes is probably right.
