Make sure to reset overflow in InlineFlowBow::computeOverflow

This patch fixes a bug where the overflow rects were not being reset
in InlineFlowBow::computeOverflow which led to a stale scroll size.
With this patch we clear the overflow rects at the beginning of
computeOverflow.

This patch is a partial merge of http://trac.webkit.org/changeset/165602
by Manish R Gurnaney <m.gurnaney@samsung.com>.

This patch also includes a few cleanups not present in the original:
1) setLayoutOverflow and setVisualOverflow were unnecessarily
public functions. They have been redeclared as private.
2) setLayoutOverflow and setVisualOverflow did redundant work to
compute the frameBox. This patch factors out the frameBox calculation
into setOverflowFromLogicalRects so it is only done once.
3) An assert has been added that knownToHaveNoOverflow() implies
m_overflow is null.

BUG=316549

Review URL: https://codereview.chromium.org/199523005

git-svn-id: svn://svn.chromium.org/blink/trunk@169321 bbb929c8-8fbe-4397-9dbb-9b2b20218538