Fixes <http://webkit.org/b/52023> WKView accumulates pixel garbage
before web process has had a chance to draw anything (if Aero is
disabled)

Reviewed by Jon Honeycutt.

* UIProcess/ChunkedUpdateDrawingAreaProxy.cpp:
(WebKit::ChunkedUpdateDrawingAreaProxy::paint):
* UIProcess/ChunkedUpdateDrawingAreaProxy.h:
* UIProcess/DrawingAreaProxy.h:
* UIProcess/LayerBackedDrawingAreaProxy.cpp:
(WebKit::LayerBackedDrawingAreaProxy::paint):
* UIProcess/LayerBackedDrawingAreaProxy.h:
* UIProcess/TiledDrawingAreaProxy.cpp:
(WebKit::TiledDrawingAreaProxy::paint):
* UIProcess/TiledDrawingAreaProxy.h:
* UIProcess/mac/ChunkedUpdateDrawingAreaProxyMac.mm:
(WebKit::ChunkedUpdateDrawingAreaProxy::platformPaint):
* UIProcess/qt/ChunkedUpdateDrawingAreaProxyQt.cpp:
(WebKit::ChunkedUpdateDrawingAreaProxy::platformPaint):
* UIProcess/win/ChunkedUpdateDrawingAreaProxyWin.cpp:
(WebKit::ChunkedUpdateDrawingAreaProxy::platformPaint):
* UIProcess/win/LayerBackedDrawingAreaProxyWin.cpp:
(WebKit::LayerBackedDrawingAreaProxy::paint):
Changed these functions to return a boolean indicating whether we
actually painted anything.

* UIProcess/win/WebView.cpp:
(WebKit::WebView::onPaintEvent): Fill with white (and don't call
didDraw) when the DrawingAreaProxy isn't able to paint.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75267 268f45cc-cd09-0410-ab3c-d52691b4dbfc