For some reason, on Mac setup_tools won't create the installation directory if
it doesn't exist, while it appears to do so on Linux and Windows.

BUG=
TEST=


Review URL: http://codereview.chromium.org/9323030

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@120331 0039d316-1c4b-4281-b951-d872f2087c98