Reviewed by Sam.

        <rdar://problem/5575812> REGRESSION:When using absolute positioning 
        with overflow:auto div, WebKit seems to add an additional 15px

        * rendering/RenderBox.cpp:
        (WebCore::RenderBox::containingBlockWidthForPositioned): We need to 
        subtract off the vertical scrollbar width too.

LayoutTests:

        Reviewed by Sam.

        New test for <rdar://problem/5575812> REGRESSION:When using 
        absolute positioning with overflow:auto div, WebKit seems to add an 
        additional 15px
        * fast/overflow/overflow-auto-position-absolute.html: Added.
        * platform/mac/fast/overflow/overflow-auto-position-absolute-expected.checksum: Added.
        * platform/mac/fast/overflow/overflow-auto-position-absolute-expected.png: Added.
        * platform/mac/fast/overflow/overflow-auto-position-absolute-expected.txt: Added.
        
        Test with changed results. The measurements in the rtl example now 
        more closely mirror the ltr example.
        * platform/mac/fast/overflow/unreachable-overflow-rtl-bug-expected.txt:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@27499 268f45cc-cd09-0410-ab3c-d52691b4dbfc