Fix for <rdar://problem/4537274> HiDPI: <canvas> needs to apply a 
        scale. <canvas> needs to scale itself along with whatever scale 
        factor is set on the window.

        * html/HTMLCanvasElement.cpp:
        (WebCore::HTMLCanvasElement::createDrawingContext): Get the scale 
        factor and use it to create a larger buffer and to scale the 
        CGContext.
        * platform/Widget.h: Provide a new function that returns the user 
        space scale factor.
        * platform/mac/WidgetMac.mm:
        (WebCore::Widget::scaleFactor): Get the scale factor from the 
        window and return it.
        * platform/win/WidgetWin.cpp:
        (WebCore::Widget::scaleFactor): Just return 1.0f for now.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@14711 268f45cc-cd09-0410-ab3c-d52691b4dbfc