[iOS WebKit2] REGRESSION (r168493): Canvasmark 2013 benchmark always hits the RELEASE...

[iOS WebKit2] REGRESSION (r168493): Canvasmark 2013 benchmark always hits the RELEASE_ASSERT in RemoteLayerTreeDrawingArea::flushLayers()
https://bugs.webkit.org/show_bug.cgi?id=132783
<rdar://problem/16877326>

Reviewed by Anders Carlsson.

* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::BackingStoreFlusher::flush):
There's a race window between the BackingStoreFlusher sending the new
layer tree to the UI process and updating m_hasFlushed where we can
get a reply back in (on the Web process main thread) from the UI process
that we committed the new layer tree (didUpdate).
This will cause the RELEASE_ASSERT in flushLayers() to fire incorrectly.
Since the RELEASE_ASSERT is there only to avoid painting garbage, move
our code to set m_hasFlushed=true to after we finish calling CGContextFlush
on all of the contexts, instead of after we send the layer tree commit message,
which will remove the race entirely.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168589 268f45cc-cd09-0410-ab3c-d52691b4dbfc