Google Fiber IPv6 has not been very reliable with Cisco ASA pulling prefix delegation. If you have a static block of IP Addresses, this configuration has been successful. It has stopped working a few times and changing the client pd name resolved the last issue.
interface GigabitEthernet0/0
description Google Fiber
nameif outside
security-level 0
dhcp client route distance 10
ip address dhcp setroute
ipv6 address autoconfig
ipv6 address dhcp default
ipv6 enable
ipv6 nd suppress-ra
ipv6 nd managed-config-flag
ipv6 nd other-config-flag
ipv6 dhcp client pd hint xxxx:xxxx:xxxx:xxxx::/56 ( Replace with your IPv6 Block)
ipv6 dhcp client pd hint ::/56
ipv6 dhcp client pd yourcompanyname-gf