[Win] Enable Encrypted Media Support

[Win] Enable Encrypted Media Support
https://bugs.webkit.org/show_bug.cgi?id=137710
<rdar://problem/18652699>

Reviewed by Jer Noble.

Source/WebCore:

This feature is covered by existing media tests.

* platform/graphics/avfoundation/cf/MediaPlayerPrivateAVFoundationCF.cpp:
(WebCore::destroyAVFWrapper): Remove redundant ASSERT.
* platform/graphics/ca/win/PlatformCALayerWinInternal.cpp:
(displayOnMainThreadContext::displayOnMainThreadContext): Added.
(redispatchOnMainQueue): Added.
(PlatformCALayerWinInternal::displayCallback): Check if a display
call is happening on a background queue, and re-dispatch on the
main thread if necessary.

WebKitLibraries:

* win/tools/vsprops/FeatureDefines.props: Activate the
ENCRYPTED_MEDIA_V2 flag on Windows.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@174698 268f45cc-cd09-0410-ab3c-d52691b4dbfc