Let MSan and LSan bots download Clang despite of the --mac-only flag.

BUG=178409
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/134833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244707 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244707