https://bugs.webkit.org/show_bug.cgi?id=124165
<rdar://problem/15199056>

Reviewed by Anders Carlsson.

Implement page overlays, very similar to how they work in TiledCoreAnimationDrawingArea,
but manipulating the PlatformCALayer tree instead of the CALayer tree directly.

* WebProcess/WebPage/mac/RemoteLayerTreeContext.h:
* WebProcess/WebPage/mac/RemoteLayerTreeContext.mm:
(WebKit::RemoteLayerTreeContext::outOfTreeLayerWasAdded):
(WebKit::RemoteLayerTreeContext::outOfTreeLayerWillBeRemoved):
(WebKit::RemoteLayerTreeContext::flushLayers):
Add a list of out-of-tree GraphicsLayers which are flushed in addition
to the page's GraphicsLayer tree, and functions to manipulate this list.

* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
Make RemoteLayerTreeDrawingArea a GraphicsLayerClient, so we can get callbacks
from the GraphicsLayers that drive our page overlays.
Override the relevant page overlay functions.
Add storage for the root layer, view size, and a map of PageOverlay->GraphicsLayer.

* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::setRootCompositingLayer):
Store the root GraphicsLayer for later use.

(WebKit::RemoteLayerTreeDrawingArea::updateGeometry):
Update the size of the page overlays when the DrawingArea's geometry changes.

(WebKit::RemoteLayerTreeDrawingArea::updatePreferences):
Update the page overlay settings when necessary.

(WebKit::RemoteLayerTreeDrawingArea::didInstallPageOverlay):
Create a GraphicsLayer for our page overlay, set it up, and parent it.
Inform the RemoteLayerTreeContext that it will need to flush this GraphicsLayer,
since it's not a part of the page's GraphicsLayer tree.

(WebKit::RemoteLayerTreeDrawingArea::didUninstallPageOverlay):
Tear down our page overlay's GraphicsLayer.

(WebKit::RemoteLayerTreeDrawingArea::setPageOverlayNeedsDisplay):
(WebKit::RemoteLayerTreeDrawingArea::setPageOverlayOpacity):
(WebKit::RemoteLayerTreeDrawingArea::paintContents):
(WebKit::RemoteLayerTreeDrawingArea::deviceScaleFactor):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159079 268f45cc-cd09-0410-ab3c-d52691b4dbfc