LayerTreeCoordinator: Make sure that dependent layers are deleted before their directly composited image
https://bugs.webkit.org/show_bug.cgi?id=100495

Reviewed by Kenneth Rohde Christiansen.

Since r128980 layers removals are delayed until we flush the layer tree.
This causes a crash if such a layer had a directly composited image since
the image is now deleted while the layer is still active in the UI process
and tries to access a deleted backing store.

Fix it by delaying the directly composited image removal as well.

Fixes a crash in tst_QQuickWebView::basicRenderingSanity since showWebView is
called first and loads direct-image-compositing.html.

* UIProcess/CoordinatedGraphics/LayerTreeRenderer.h:
(LayerTreeRenderer): Remove unused variable.
* WebProcess/WebPage/CoordinatedGraphics/LayerTreeCoordinator.cpp:
(WebKit::LayerTreeCoordinator::flushPendingLayerChanges):
(WebKit::LayerTreeCoordinator::releaseImageBackingStore):
* WebProcess/WebPage/CoordinatedGraphics/LayerTreeCoordinator.h:
(LayerTreeCoordinator):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@132640 268f45cc-cd09-0410-ab3c-d52691b4dbfc