Must be funny working at Google and knowing the inside scoop (watching people speculate). So, here is my wild speculation about a similar secret project. There was a talk given by Dick Sites (from Google) [1, 2], where he talks about performance monitoring across the Google fleet. Very technical and useful if you are into monitoring (highly recommend it). A large part of his talk is dedicated to issues and limitations with off-the-shelf CPUs. Given that AWS has custom chips (talked about at Invent a few years back), why wouldn't Google solve this issue too, they have the talent and money.
It's already been posted in this thread that it's just a smart NIC evolved from existing Myricom NICs. (Of course, my coworker was just telling me a story about how John Cocke hid his RISC processor in a printer project.)
And Amazon's "custom" CPUs are 100 MHz faster Xeons.
[1] http://www.pdl.cmu.edu/SDI/2015/slides/DatacenterComputers.p...
[2] https://vimeo.com/121396406