Add a content shadow layer to the render layer compositor

Add a content shadow layer to the render layer compositor
https://bugs.webkit.org/show_bug.cgi?id=78133
<rdar://problem/10797742>

Reviewed by Beth Dakin.

Have the render layer compositor optionally create a content shadow layer,
and add a ScrollbarTheme::setUpContentShadowLayer member function that subclasses
can use to set content shadow properties.

* platform/mac/ScrollbarThemeMac.mm:
(WebCore::ScrollbarThemeMac::setUpContentShadowLayer):
Set the layer properties once, and set the shadow path on every call, since we know that this
function will be called every time the size of the content shadow layer changes.

* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateRootLayerPosition):
Reposition the content shadow layer, and call ScrollbarTheme::setUpContentShadowLayer if the size changes.

(WebCore::RenderLayerCompositor::requiresContentShadowLayer):
Add new helper function.

(WebCore::RenderLayerCompositor::updateOverflowControlsLayers):
Create a content shadow layer if needed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107119 268f45cc-cd09-0410-ab3c-d52691b4dbfc