I did some further digging. It appears that the webkit developers believed the Qt developers were already introducing a hard dependency on ICU, and therefore moved from using Qt Unicode functions to ICU for consistency with other platforms:
https://bugs.webkit.org/show_bug.cgi?id=76821

Independently of Qt, there is interest in reducing the large overhead that ICU introduces to webkit:
http://mac-os-forge.2317878.n4.nabbl...-td219912.html
And there's an abstraction layer which theoretically enables switching to something else, though nothing seems to be forthcoming.