When the web page transparency changes, DrawingAreaImpl needs to know about it
https://bugs.webkit.org/show_bug.cgi?id=115062

Reviewed by Tim Horton.

The non-composited contents layer owned by LayerTreeHostMac needs to
be marked as opaque or not depending on whether the WebPage draws
a background (or transparent background). It computed this at creation,
but did not dynamically update the state. Fix it so it does.

This fixes cases of garbage pixels in the Web Inspector toolbar area
in some configurations.

* WebProcess/WebPage/DrawingAreaImpl.cpp:
(WebKit::DrawingAreaImpl::pageBackgroundTransparencyChanged):
* WebProcess/WebPage/DrawingAreaImpl.h:
(DrawingAreaImpl):
* WebProcess/WebPage/LayerTreeHost.h:
* WebProcess/WebPage/mac/LayerTreeHostMac.h:
* WebProcess/WebPage/mac/LayerTreeHostMac.mm:
(WebKit::LayerTreeHostMac::pageBackgroundTransparencyChanged):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149004 268f45cc-cd09-0410-ab3c-d52691b4dbfc