Compute margins correctly when child's margin-box exceeds available width

Per CSS 2.1 we should use the margin box rather than the border box when
deciding if a non-replaced block flow element exceeds the available width.

BUG=331454
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/268853004

git-svn-id: svn://svn.chromium.org/blink/trunk@174236 bbb929c8-8fbe-4397-9dbb-9b2b20218538