[iOS] Enable media element volume on iPad

[iOS] Enable media element volume on iPad
https://bugs.webkit.org/show_bug.cgi?id=193745
<rdar://problem/47452297>

Reviewed by Jer Noble.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::setVolume):
(WebCore::HTMLMediaElement::mediaPlayerVolumeChanged):
(WebCore::HTMLMediaElement::updateVolume):

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

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@240434 268f45cc-cd09-0410-ab3c-d52691b4dbfc