For Mac, we're depending on having both ios+mac for target_os.

R=agable@chromium.org

Review URL: https://codereview.chromium.org/753123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293085 0039d316-1c4b-4281-b951-d872f2087c98