Adds support for passing target_os_only to the chromium recipe, and adds
"fetch ios" as an alias for passing both target_os=['ios'] and
target_os_only=True as described in the iOS build instructions page.

BUG=None
R=agable@chromium.org, cmp@chromium.org

Review URL: https://codereview.chromium.org/21428002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@215891 0039d316-1c4b-4281-b951-d872f2087c98