I'm still having this issue. I ran EtreCheck, which reported that I was having frequent crashes of com.apple.WebKit.WebContent. I've found ways to resolve this with Safari (which hasn't caused any system reboots), but haven't found anything to try to address it with Opera for Mac. I've installed the latest updated for Opera and done a total wipe and reinstall of my system and the frequent system reboots while using Opera persists.
Any pointers to something to try to resolve this (other than stop using Opera)?