In bug #251977 we switched from 0 to 1 painting threads, thinking that although using multiple painting threads is not safe, using exactly one might be. I think this is not true because bug #247628 seems to be caused by use of the painting thread. Let's go back to 0 painting threads?
Pull request: https://github.com/WebKit/WebKit/pull/10830
Closing as Carlos Garcia has an attempted fix in bug #247628.