Revert "Bug 906316 - Don't download the xulrunner SDK each time we change a branch if their configuration are different r=yurenju,asuth,ochameau"

This reverts commit 106001c4e0ab9253001bed815c03f46b8dd046f0.
Merge pull request #12484 from lightsofapollo/revert-xpcshell-nice

Revert "Bug 906316 - Don't download the xulrunner SDK each time we chang...