Reviewed by Eric Seidel.

        Create LayerOffset class
        https://bugs.webkit.org/show_bug.cgi?id=60490

        Adding a LayerOffset class that will replace existing usage of tx/ty.
        LayerOffset is necessary since these values are commonly used as both
        a size and a point. Updating paintIntoRect to use this new class.

        No new tests since this doesn't change any behavior.

        * GNUmakefile.list.am:
        * WebCore.gypi:
        * WebCore.pro:
        * WebCore.vcproj/WebCore.vcproj:
        * WebCore.xcodeproj/project.pbxproj:
        * page/FrameView.cpp:
        (WebCore::FrameView::paintScrollCorner):
        * platform/graphics/LayerOffset.h: Added.
        (WebCore::LayerOffset::LayerOffset):
        (WebCore::LayerOffset::x):
        (WebCore::LayerOffset::y):
        (WebCore::LayerOffset::setX):
        (WebCore::LayerOffset::setY):
        (WebCore::LayerOffset::toSize):
        (WebCore::LayerOffset::toPoint):
        (WebCore::operator+=):
        (WebCore::operator-=):
        (WebCore::operator+):
        (WebCore::operator-):
        (WebCore::operator==):
        (WebCore::operator!=):
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::paintScrollCorner):
        (WebCore::RenderLayer::paintResizer):
        * rendering/RenderScrollbar.cpp:
        (WebCore::RenderScrollbar::paintPart):
        * rendering/RenderScrollbarPart.cpp:
        (WebCore::RenderScrollbarPart::paintIntoRect):
        * rendering/RenderScrollbarPart.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@86195 268f45cc-cd09-0410-ab3c-d52691b4dbfc