Use correct container width as base for percentage margins.

Blocks establishing a new BFC need to avoid floats in its parent BFC. Also,
computing used values for auto margins for such blocks is affected by the
available line width constrained by these floats. However, percentage sized
margins must be calculated based on the full containing block width, not the
available line width.

R=jchaffraix@chromium.org
BUG=348927

Review URL: https://codereview.chromium.org/185723005

git-svn-id: svn://svn.chromium.org/blink/trunk@169030 bbb929c8-8fbe-4397-9dbb-9b2b20218538