Fix gyptest-app for Xcode 5.0.2

With Xcode 5.0.0 or later, the DTSDKName of Info.plist is always
set, even if SDKROOT is not defined in the Xcode project.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/83463003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1808 78cadc50-ecff-11dd-a971-7dbc132099af