Fix RenderBox layout overflow.

hasHorizontalLayoutOverflow and hasVerticalLayoutOverflow now simply compares the layout overflow rect to the no-overflow rect; this is both clearer and more correct than what exists now.

A small error in RenderBox::clearLayoutOverflow was also corrected; m_overflow should be cleared only if there was no visual overflow (note that the default visual overflow rect is the border box, not the no-overflow box).

BUG=254002

Review URL: https://chromiumcodereview.appspot.com/17722002

git-svn-id: svn://svn.chromium.org/blink/trunk@155339 bbb929c8-8fbe-4397-9dbb-9b2b20218538