[iOS] Fixed items are sometimes clipped after rubber-banding
https://bugs.webkit.org/show_bug.cgi?id=132851
<rdar://problem/16870790>

Reviewed by Benjamin Poulain.

On iOS fixed-position layers would get clipped to the document rect,
but with rubber-banding, we can now have a custom fixed postion rect
that extends outside the document rect.

Another issue was that we would un-composite fixed elements sometimes
when scrolling fast, again because they could fall outside of the document rect.

A final issue was that pinching could reveal parts of fixed elements that should
lie outside the viewport, rather than clipping the layers.

Fix by converting both call points to use viewportConstrainedVisibleContentRect()
rather than viewportConstrainedExtentRect(). On non-iOS platforms these are
the same, but on iOS viewportConstrainedVisibleContentRect() uses the
custom fixed position rect, which is the correct rect to intersect with.

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateCompositedBounds):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::requiresCompositingForPosition):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@168670 268f45cc-cd09-0410-ab3c-d52691b4dbfc
[Win] Unreviewed build fix after r168668.

* WebKit.vcxproj/WebKitExportGenerator/WebKitExports.def.in: Add
missing export symbols.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@168671 268f45cc-cd09-0410-ab3c-d52691b4dbfc