Introduce a compositing trigger for scrollable frames
https://bugs.webkit.org/show_bug.cgi?id=105573

Reviewed by James Robinson.
Patch by Antonio Gomes <a1.gomes@sisa.samsung.com>

Source/WebCore:

Make requiresCompositingForScrollableFrame to check the
newly added compositing trigger instead of the associated setting.

It introduces no have behavior change, so no new tests. Patch basically
does some preparation work for adding support to composite scrollable inner
frames independently from "force compositing mode" bit.

* page/ChromeClient.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::cacheAcceleratedCompositingFlags):
(WebCore::RenderLayerCompositor::requiresCompositingForScrollableFrame):

Source/WebKit/chromium:

Set the newly added scrollable-inner-frames bit to compositing-trigger
bitset in accordance to the respective setting value.

* src/ChromeClientImpl.cpp:
(WebKit::ChromeClientImpl::allowedCompositingTriggers):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139053 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Make RenderLayer::updateNeedsCompositedScrolling scrollbars agnostic
https://bugs.webkit.org/show_bug.cgi?id=95494

Reviewed by James Robinson.
Patch by Antonio Gomes <a1.gomes@sisa.samsung.com>

Some ports (including Qt-wk1 and BlackBerry) allow disabling
scrollbars at FrameView creation level. That said, cheking for the
scrollbars presence in order to determine if we should promote a
RenderLayer to use composited scrolling is not ideal, as done in
RenderLayerCompositor::updateNeedsCompositedScrolling()

Instead, this patch makes the RenderLayer class to query its FrameView
if it has been cached as scrollable by the later.

Least, patch also takes this opportunity to remove RenderLayer::allowsScrolling,
which became unused.

No new tests added, since there is no *practical* behavior change: ports
like Chromium and others who currently check for the scrollbars presence
in order to determine either to promote or not layers to benefit of composited
scrolling should not be affected.

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::updateNeedsCompositedScrolling):
* rendering/RenderLayer.h:
(RenderLayer):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139054 268f45cc-cd09-0410-ab3c-d52691b4dbfc