Maybe some testing of, say, a few tabs, but I'd be very surprised if a significant number of Chrome devs (or any Chrome devs, for that matter) use Chromebooks for Chrome development (maybe the Chromebook Pixel, but even that seems unlikely). The Chromebook is optimized toward content consumption; while I'm sure it's possible to do serious programming on ChromeOS (just as it's possible to do serious programming on Android or (jailbroken) iOS if you really wanted to), it's not going to be nearly as productive as a high end workstation (portable or stationary) with 16+GB of RAM and a quad+ core processor.
Many Google engineers (including folks like me outside of Chrome) use Chromebooks as our primary laptops. In my case that also (mostly) makes my Chromebook Pixel my primary computer. I average 25-100 tabs, many of them doing 'heavy' things like editing large Google Docs or running Inbox.
I do have a beefy dev desktop that I access via ssh, although frankly it mostly exists for running emacs for the pedestrian task of actually typing out bits of code.
I remember having a few Google engineers coming to my uni to do a talk - afterwards they mentioned that internally they've been doing a large amount of programming work using internal tools on Chromebooks.
