Reviewed by Dan Bernstein, Darin Adler.

        Remove addScrolledContentOffset/subtractScrolledContentOffset
        https://bugs.webkit.org/show_bug.cgi?id=35225

        Remove RenderLayer's addScrolledContentOffset() and subtractScrolledContentOffset()
        methods, and instead use the existing scrolledContentOffset(), and use
        IntSize and IntPoint instead of lots of x, y variables.

        Added new IntPoint toPoint(const IntSize&) method as a convenience to convert a size to a point,
        which is needed in a few places.

        * page/EventHandler.cpp:
        (WebCore::EventHandler::handleDrag): Use the new toPoint() convenience.
        * platform/graphics/IntPoint.h:
        (WebCore::toPoint): New convenience method to convert an IntSize to an IntPoint.
        * rendering/LayoutState.cpp:
        (WebCore::LayoutState::LayoutState):
        * rendering/RenderBlock.cpp:
        (WebCore::RenderBlock::layoutBlock):
        (WebCore::RenderBlock::paintObject):
        (WebCore::RenderBlock::selectionGapRectsForRepaint):
        (WebCore::RenderBlock::nodeAtPoint):
        (WebCore::RenderBlock::offsetForContents):
        * rendering/RenderBoxModelObject.cpp:
        (WebCore::RenderBoxModelObject::paintFillLayerExtended):
        * rendering/RenderInline.cpp:
        (WebCore::RenderInline::clippedOverflowRectForRepaint):
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::updateLayerPosition):
        * rendering/RenderLayer.h:
        (WebCore::RenderLayer::size):
        * rendering/RenderObject.cpp:
        (WebCore::RenderObject::computeRectForRepaint):
        * rendering/TextControlInnerElements.cpp:
        (WebCore::RenderTextControlInnerBlock::positionForPoint):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@55090 268f45cc-cd09-0410-ab3c-d52691b4dbfc