[Mac] naturalSize not updated when preload=metadata

[Mac] naturalSize not updated when preload=metadata
https://bugs.webkit.org/show_bug.cgi?id=132994

Reviewed by Eric Carlson.

Source/WebCore:
Test: media/video-load-preload-metadata-naturalsize.html

When preload=metadata, we never create an AVPlayerItem, and therefore never receive a
presentationSize KVO. When an AVAsset informs us that its tracks have changed, and there
is no AVPlayerItem, recalculate presentation size based on the first video track's natural
size.

* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::tracksChanged):
* platform/graphics/avfoundation/objc/MediaPlayerPrivateMediaSourceAVFObjC.mm:
(WebCore::MediaPlayerPrivateMediaSourceAVFObjC::seekInternal):

LayoutTests:
* media/video-load-preload-metadata-naturalsize-expected.txt: Added.
* media/video-load-preload-metadata-naturalsize.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168979 268f45cc-cd09-0410-ab3c-d52691b4dbfc