Created attachment 166401 [details] /var/log/kern.log containing the trace. Acer chromebook c720 running Ubuntu 14.04 with mainline kernel from http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.19-vivid/. Upgraded to 3.19.0 this morning and after about couple hours of normal (text editing, web browsing) usage, system froze when I clicked a link in firefox. Laptop did not react to keyboard or mouse anymore. Didn't try to take remote connection from another computer. After poweroff/on everything was ok and I didn't experience more freezes during the day. Checked /var/log/kern.log and found the trace. Didn't experience freezes with 3.18.[5,6].
This just happened again. Browsing with firefox with seven tabs open. I was able to ssh into laptop with phone. dmesg attached. "sudo service lightdm restart" said that it stopped the service, but after that nothing. Had to stop it with ctrl-c. After that "sudo reboot" brought the laptop back again.
Created attachment 166441 [details] dmesg output just after the freeze
https://bugs.freedesktop.org/show_bug.cgi?id=89085