Small fix for package_management on Mac OS X.

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

package_management.py

@@ -472,7 +472,7 @@ def UpdateSiteDirectory():
     return True
 
   try:
-    AddSiteDirectory(SITE_DIR)
+    EnsureSiteDirectory(SITE_DIR)
     import pkg_resources
 
     # Determine if any packages actually need installing.