Ah - my mistake. I was operating on the assumption the Blink and WebKit teams were exchanging patches regularly.
That's a dang shame then :/
Apple's a big company - but we saw how they mishandled their own first-party Maps service after divorcing from Google - I can see Apple's Safari potentially falling behind badly if they can't keep-up with Google's work on Blink.
They already are; there are some pretty glaring bugs/missing features in webkit nowadays.
In fact, I can't think of any webkit developments that positively surprised me the past few years; development seems glacial, at best. A list of somewhat notable stuff chromium and gecko have that webkit is still missing:
Stuff because it's better to make your devs pay licenses for no good reason: