Fix overhang computation for Mac overscroll bounce

Compute the overscroll amount computation using the
minimum/maximumScrollPosition function, instead of
replicating that code. Note that the difference is that
new functions' maximum scroll positions are clamped
to zero (which fixes the shuddering on bounce bug).

Note that there is still a bug where the contents rectangle will
be inappropriately smaller than the view. Add comments to
FrameView to indicate this, and refactor the scrollbar creation
code to provide a function that can be used to determine when
an extra layout should be performed.

BUG=269692

Review URL: https://codereview.chromium.org/129793004

git-svn-id: svn://svn.chromium.org/blink/trunk@164815 bbb929c8-8fbe-4397-9dbb-9b2b20218538