[Mac] 9 WebGL conformance failures after r159518.

[Mac] 9 WebGL conformance failures after r159518.
https://bugs.webkit.org/show_bug.cgi?id=124608

Reviewed by Dean Jackson.

Once we removed the CIImage drawing path, there was no longer any reason to flip the
CGImageRef before drawing to the provided GraphicsContext.

* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::paintWithVideoOutput):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@159525 268f45cc-cd09-0410-ab3c-d52691b4dbfc