Make ninja the default build system for build-webkit --chromium on mac
https://bugs.webkit.org/show_bug.cgi?id=106737

Reviewed by Eric Seidel.

* Scripts/update-webkit-chromium:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140260 268f45cc-cd09-0410-ab3c-d52691b4dbfc