Fix crash due to animationDidEnd called on deallocated RemoteLayerTreeHost

Fix crash due to animationDidEnd called on deallocated RemoteLayerTreeHost
https://bugs.webkit.org/show_bug.cgi?id=148442
<rdar://problem/21609257>

Reviewed by Tim Horton.

A PlatformCAAnimationRemote's backpointer to a deallocated RemoteLayerTreeHost is not
invalidated when its host removes its reference to it.

* UIProcess/mac/RemoteLayerTreeHost.mm:
(WebKit::RemoteLayerTreeHost::layerWillBeRemoved): Invalidate a backpointer from the
    PlatformCAAnimationRemotes to the RemoteLayerTreeHost.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@188991 268f45cc-cd09-0410-ab3c-d52691b4dbfc