https://bugs.webkit.org/show_bug.cgi?id=125189
<rdar://problem/15541789>

Reviewed by Anders Carlsson.

* WebProcess/WebPage/mac/RemoteLayerTreeContext.h:
* WebProcess/WebPage/mac/RemoteLayerTreeContext.mm:
(WebKit::RemoteLayerTreeContext::forceRepaint):
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::forceRepaint):
Implement WebProcess-synchronous force repaint.
We don't need the async variant because it is only needed
to synchronize with the WebProcess-side threaded scrolling
tree, which is not a component of the remote layer tree model.
The UI process will not handle the callback until after
RemoteLayerTreeHost::commit is complete, ensuring that the commit
is actually done.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160052 268f45cc-cd09-0410-ab3c-d52691b4dbfc