Move m_scrollOffset to RenderLayerScrollableArea

This change pulls one member (m_scrollOffset) and its associated
code off RenderLayer to the new class.

While moving the code, added an helper to access the RenderBox from
RenderLayerScrollableArea.

BUG=260899

Review URL: https://chromiumcodereview.appspot.com/20071004

git-svn-id: svn://svn.chromium.org/blink/trunk@156599 bbb929c8-8fbe-4397-9dbb-9b2b20218538