Google has not yet transitioned from the U2F API to the Web Authentication API for login so no, not yet. (We're working on it, but there's quite a lot of moving pieces.)
Also, as another comment noted, only CTAP2 is supported for the host to authenticator API, and only the newest Yubico tokens support CTAP2.
Hopefully someone more knowledgeable will chime in as I'm waiting for FIDO U2F support as well but I think CTAP2 is the browser-to-user layer of FIDO.
edit: just tested it on Mojave with gmail and fastmail and it doesn't recognize that the Yubikey has been inserted, so looks like support is not there out-of-the-box
As far as I understand it, it is. That said, I tried to use this for a WebAuthn Django library I'm trying to write and it wouldn't work, as pages wouldn't render at all.
You need to enable it in Develop->Experimental Features, but also you need to be sure you have a CTAP2 key. "U2F" keys are all CTAP1, which is not supported yet. Yubi's 5 series are CTAP2, as are the blue keys with a 2 printed on them.
Also, as another comment noted, only CTAP2 is supported for the host to authenticator API, and only the newest Yubico tokens support CTAP2.