Merge 99485 - Source/WebCore: Create a separate setting for compositing of for scrollable [i]frames

Merge 99485 - Source/WebCore: Create a separate setting for compositing of for scrollable [i]frames
that forceCompositingMode can be used without turning frames into composited layers.
https://bugs.webkit.org/show_bug.cgi?id=71714

Reviewed by James Robinson.

* page/Settings.cpp:
(WebCore::Settings::Settings):
* page/Settings.h:
(WebCore::Settings::setAcceleratedCompositingForScrollableFramesEnabled):
(WebCore::Settings::acceleratedCompositingForScrollableFramesEnabled):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::cacheAcceleratedCompositingFlags):
* testing/Internals.cpp:
(WebCore::Internals::setEnableCompositingForScrollableFrames):
* testing/Internals.h:
* testing/Internals.idl:

LayoutTests: [chromium] Make compositing for scrollable [i]frames a setting separate from
forceCompositingMode.
https://bugs.webkit.org/show_bug.cgi?id=71714

Adjust the force-compositing-mode layout tests to explicitly set the new flag.

Reviewed by James Robinson.

* platform/chromium/compositing/force-compositing-mode/force-composite-empty.html:
* platform/chromium/compositing/force-compositing-mode/no-overflow-iframe-layer.html:
* platform/chromium/compositing/force-compositing-mode/overflow-hidden-iframe-layer.html:
* platform/chromium/compositing/force-compositing-mode/overflow-iframe-enter-compositing.html:
* platform/chromium/compositing/force-compositing-mode/overflow-iframe-layer.html:
* platform/chromium/compositing/force-compositing-mode/overflow-iframe-leave-compositing.html:


TBR=vangelis@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=99758