[webkit-dev] Introducing a minimum ICU version for WebKit

[Michael Catanzaro]

Any objections to uploading a bundled ICU 60 under Source/ThirdParty?

Seems easier than forcing downstreams to work out bundling themselves. 
Most major distros will just stop providing WebKit security updates if 
we don't bundle it for them. E.g. this is sure to kill Ubuntu's current 
long streak of providing our updates to Ubuntu 16.04.