Dropdowns on http://www.exploratorium.edu don't show anything
https://bugs.webkit.org/show_bug.cgi?id=115991

Source/WebCore:

Reviewed by Dan Bernstein.

We can't optimize away the backing store of a layer by saying that
it paints into a composited ancestor if its composited bounds are not contained
by that ancestor.

Test: compositing/backing/no-backing-for-clip-overhang.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateGraphicsLayerGeometry): Pass in our
previously computed composited bounds relative to our composited ancestor,
and its composited bounds.
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::requiresOwnBackingStore): If the ancestor's
composited bounds don't contain the layer's composited bounds, the layer
needs its own backing store.
* rendering/RenderLayerCompositor.h:

LayoutTests:

Reviewed by Dan Bernstein.

Layer tree dump test with an overflow:hidden that projects outside its
ancestor, and is forced to composite by a composited child.

* compositing/backing/no-backing-for-clip-overhang-expected.txt: Added.
* compositing/backing/no-backing-for-clip-overhang.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149969 268f45cc-cd09-0410-ab3c-d52691b4dbfc