[Win] Avoid deadlock when interacting with some AVFoundationCF content

[Win] Avoid deadlock when interacting with some AVFoundationCF content
<rdar://problem/15482977> and https://bugs.webkit.org/show_bug.cgi?id=124752

Prevent deadlock caused by conflict over the "mapLock" mutex. Notification handling in the file,
which modify assets and make other changes, are required to happen on the main thread. This
patch enforces this requirement.

Reviewed by Eric Carlson.

* platform/graphics/avfoundation/cf/MediaPlayerPrivateAVFoundationCF.cpp:
(WebCore::NotificationCallbackData::NotificationCallbackData): Added
(WebCore::AVFWrapper::processNotification): Moved logic from 'notificationCallback', which was
sometimes happening on a background thread.
(WebCore::AVFWrapper::notificationCallback): Dispatch calls to main thread.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@159691 268f45cc-cd09-0410-ab3c-d52691b4dbfc