Make ninja the default build system on Linux for build-webkit --chromium
https://bugs.webkit.org/show_bug.cgi?id=104434

Reviewed by Eric Seidel.

* Scripts/update-webkit:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139557 268f45cc-cd09-0410-ab3c-d52691b4dbfc