In the case of Blink at least, they basically own almost all their dependencies. From my memory WebKit is the same. It makes sense that they would end up that way given their history - KHTML -> Safari -> Chrome all target multiple runtime environments and in the case of Safari especially they were targeting an environment without a bunch of existing unix/linux history behind it.

In comparison historically Firefox has been more aggressive about pulling in 3rd-party dependencies like cairo, ANGLE, etc.

(You could count ANGLE as a 3rd-party dependency for Blink, I suppose, but I don't because AFAIK it's a Google project originally)