Some regression tests require enabling accelerated compositing. The TestRunner for WK2 supports it but not WK1. So this message 'TestRunner::overridePreference tried to override unknown preference 'WebKitAcceleratedCompositingEnabled'.' comes out while running such tests.
Created attachment 211179 [details] Patch
The bots don't really have good support for running OpenGL tests inside xvfb yet.
As martin's comment, this change would be halted until it'd be meaningful.