Add is_ios to the list of grit variables

Adds is_ios mirroring the other platforms, and makes is_macosx false
and is_posix true for iOS (matching the behavior in gyp).

BUG=https://code.google.com/p/grit-i18n/issues/detail?id=10

Originally by stuartmorgan@chromium.org, https://codereview.chromium.org/15730002/

Review URL: https://codereview.chromium.org/15747003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@123 7262f16d-afe8-6277-6482-052fa10e57b1