Thanks for the report. We, Google, are currently working to add support for other ciphers. (I was going to file a bug, until I noticed there was already one with work underway).
I'm curious to know what was the basis of this configuration decision. It seems like someone went out of their way to hamstring your encryption. Possibly someone benchmarked every combination and disabled all but the fastest. Wouldn't something like this go through a security review before hitting prod?