Fix OS_MACOS typos. Should be OS_MACOSX.

BUG=163208
TEST=none

Review URL: https://codereview.chromium.org/12829005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@189130 0039d316-1c4b-4281-b951-d872f2087c98
  virtual ~TestAutofillExternalDelegate() {}

  virtual ~TestAutofillExternalDelegate() {}
