Update the iOS builders to use iOS 6.0 SDK.
Update the iOS builders to use iOS 6.0 SDK.

Now that Xcode 4.5 has been installed on the slave we can build with the 6.0 SDK.

Review URL: https://chromiumcodereview.appspot.com/10966019

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@157838 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build