Give ViewportConstraints classes copy constructors
https://bugs.webkit.org/show_bug.cgi?id=111367

Reviewed by Beth Dakin.

Downstream code requires that ViewportConstraints objects have
copy constructors, so add them.

Also clean up some redundant initialization of stack objects
in RenderLayerCompositor.

No behavior change.

* page/scrolling/ScrollingConstraints.h:
(WebCore::ViewportConstraints::ViewportConstraints):
(ViewportConstraints):
(WebCore::FixedPositionViewportConstraints::FixedPositionViewportConstraints):
(FixedPositionViewportConstraints):
(WebCore::StickyPositionViewportConstraints::StickyPositionViewportConstraints):
(StickyPositionViewportConstraints):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::computeFixedViewportConstraints):
(WebCore::RenderLayerCompositor::computeStickyViewportConstraints):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144687 268f45cc-cd09-0410-ab3c-d52691b4dbfc