[iOS] ignore requests to set volume
https://bugs.webkit.org/show_bug.cgi?id=135081

Reviewed by Jer Noble.

* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::setVolume): Do nothing on iOS.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171259 268f45cc-cd09-0410-ab3c-d52691b4dbfc