[iOS] Don't import 'UIKit-apps.sb' to the WebContent process sandbox

[iOS] Don't import 'UIKit-apps.sb' to the WebContent process sandbox
https://bugs.webkit.org/show_bug.cgi?id=180610
<rdar://problem/18899506>

Reviewed by Dean Jackson.

Stop including 'system.sb', and just include the portions of that sandbox that we
actually use in WebContent Process. This is the first step in some further sandbox
tightening.

* Resources/SandboxProfiles/ios/com.apple.WebKit.WebContent.sb:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225754 268f45cc-cd09-0410-ab3c-d52691b4dbfc