Rename OS(DARWIN) to OS(MACOSX).

Currently Mac OS X is the only DARWIN platform supported, so there is no need to make a distinction between OS(DARWIN) and OS(MACOSX).

A distinction between OS(DARWIN) and OS(MACOSX) would only make sense if blink would support (e.g. with a running bot) a  configuration where where OS(DARWIN) is set but OS(MAC) is not set. Such a configuration is not a project goal currently.

Review URL: https://chromiumcodereview.appspot.com/14107015

git-svn-id: svn://svn.chromium.org/blink/trunk@157410 bbb929c8-8fbe-4397-9dbb-9b2b20218538