[iOS WK2] Pages often blank on first load if page loaded by typing the URL

[iOS WK2] Pages often blank on first load if page loaded by typing the URL
https://bugs.webkit.org/show_bug.cgi?id=131665

Reviewed by Tim Horton.

The document overlay-related code in RemoteLayerTreeDrawingArea::setRootCompositingLayer()
was triggering a compositing layer flush when called with a null rootLayer, which happens
for pages going into the page cache. This would trigger a layer flush that would clobber
the root layer for the visible page, resulting in missing content.

Also, rebuildCompositingLayerTree() is called recursively and the m_documentOverlayRootLayer
was being added to (and then removed from) every single compositing layers.

Fix both these by changing to a pull model, where RenderLayerCompositor requests
the overlay layer via ChromeClient, and gets it at the end of every flush,
adding to the children of the root layer.

Source/WebCore:
* WebCore.exp.in:
* page/ChromeClient.h:
(WebCore::ChromeClient::documentOverlayLayerForFrame):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::RenderLayerCompositor):
(WebCore::RenderLayerCompositor::flushPendingLayerChanges): Put visibleRect
into a variable for ease of debugging.
(WebCore::RenderLayerCompositor::updateCompositingLayers): Asser
that we're not in the page cache (this would have caught the bug).
(WebCore::RenderLayerCompositor::appendOverlayLayers):
(WebCore::RenderLayerCompositor::rebuildCompositingLayerTree):
(WebCore::RenderLayerCompositor::setDocumentOverlayRootLayer): Deleted.
* rendering/RenderLayerCompositor.h:

Source/WebKit2:
* WebProcess/WebCoreSupport/WebChromeClient.cpp:
(WebKit::WebChromeClient::documentOverlayLayerForFrame):
* WebProcess/WebCoreSupport/WebChromeClient.h:
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::setRootCompositingLayer):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167316 268f45cc-cd09-0410-ab3c-d52691b4dbfc