Reviewed by Beth Dakin.

In order to make decisions about compositing, the m_hasVisibleDescendant bit on RenderLayers
needs to be up-to-date when RenderLayerCompositor::computeCompositingRequirements is recursing
over the RenderLayer tree.

However, was possible for computeCompositingRequirements() to hit a layer whose m_visibleDescendantStatusDirty
bit was set; we only clear this bit from collectLayers() (when updating z-order lists), and from styleChanged()
which requires that style changed on the layer itself.

Fix by always calling updateDescendantDependentFlags() from computeCompositingRequirements().

Wasn't able to easily get a reduced testcase.

* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::computeCompositingRequirements):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154417 268f45cc-cd09-0410-ab3c-d52691b4dbfc