This properly sets the DEPLOYMENT_TARGET in the generated GYP file (previously this value wa salways hardcoded to empty -- whoops!).
Sets the simulator properly. Previously when not using an explicit SDK the Xcode generator would still end up with a simulator build. This turns the flag into a tri-state so gyp_chromium can set it explicitly when an SDK is specified, but otherwise we can trigger default behavior based on Xcode/Ninja.
TBR=scottmg
Review URL: https://codereview.chromium.org/155983003
git-svn-id: http://src.chromium.org/svn/trunk/src/build@249643 4ff67af0-8c30-449e-8e8b-ad334ec8d88c