Document-relative overlays disappear after doing page-cache navigations

Document-relative overlays disappear after doing page-cache navigations
https://bugs.webkit.org/show_bug.cgi?id=135669
<rdar://problem/17929171>

Reviewed by Simon Fraser.

* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::rootLayerAttachmentChanged):
When navigating from one page to another, the document-relative overlay
layer is moved from the layer tree of the RenderLayerCompositor of the
first RenderView to the layer tree of the RenderLayerCompositor of the
new RenderView, upon layer tree construction.
When going "back" via a page cache navigation, we don't rebuild the
layer tree, and just assume that it is in a valid state.
However, the document-relative overlay layer was *moved*, and as such,
needs to be moved back. To do this, reattach the document-relative
overlay layer whenever the root layer attachment of a RenderLayerCompositor
changes, which will happen in the right order when going back to a cached page.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172179 268f45cc-cd09-0410-ab3c-d52691b4dbfc