Add testing for --force-compositing-mode to windows.internal

Add testing for --force-compositing-mode to windows.internal
https://bugs.webkit.org/show_bug.cgi?id=65777

Reviewed by Adam Barth.

.:

Add exports for Document::settings().

* Source/autotools/symbols.filter:

Source/WebCore:

Test: platform/chromium/compositing/force-compositing-mode/force-composite-empty.html

Move initial initialization to settings caching function so that RLC
picks up changes to the force compositing mode flag.

* WebCore.exp.in:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::RenderLayerCompositor):
(WebCore::RenderLayerCompositor::cacheAcceleratedCompositingFlags):
(WebCore::RenderLayerCompositor::updateCompositingLayers):
* testing/Internals.cpp:
(WebCore::Internals::setForceCompositingMode):
* testing/Internals.h:
* testing/Internals.idl:

Source/WebKit2:

Add exports for Document::settings().

* win/WebKit2.def:
* win/WebKit2CFLite.def:

LayoutTests:

Add empty page to test that the root gets a graphics layer.

* platform/chromium/compositing/force-compositing-mode/force-composite-empty-expected.txt: Added.
* platform/chromium/compositing/force-compositing-mode/force-composite-empty.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@92697 268f45cc-cd09-0410-ab3c-d52691b4dbfc