Tarball releases for additional modules are downloaded using python’s urllib. However this fails when traffic is routed through a proxy: $Tools/Scripts/test-webkitpy fails with error 400. However the proxy is correctly setup and can download the file with wget. The solution (and this solves the problem for me) is to replace urllib with urllib2, which should cause no trouble because urllib2 should be available everywhere but has better proxy support.
Created attachment 206658 [details] Patch
It looks like urllib2 is used quite a lot around the codebase in case that was giving pause for thought $ find Tools/Scripts/webkitpy -name "*.py" | xargs grep urllib2 | wc -l 132
Comment on attachment 206658 [details] Patch Clearing flags on attachment: 206658 Committed r152717: <http://trac.webkit.org/changeset/152717>
All reviewed patches have been landed. Closing bug.