Checker: Value stored to 'kernResult' is never read (LayoutTestHelper.m:164)
<http://webkit.org/b/135096>

Reviewed by Alexey Proskuryakov.

* DumpRenderTree/mac/LayoutTestHelper.m:
(lockDownDiscreteGraphics): Check the return values of
IOServiceOpen() and IOObjectRelease(), and log if not
KERN_SUCCESS.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171277 268f45cc-cd09-0410-ab3c-d52691b4dbfc