Margins should expand/shrink an element's width if no float constrains it

This is a regression from https://src.chromium.org/viewvc/blink?revision=175325&view=revision.
I didn't consider the case where there are floats in the same formatting
context but they don't actually overlap with the element so have no effect
on its width.

BUG=413202

Review URL: https://codereview.chromium.org/570933002

git-svn-id: svn://svn.chromium.org/blink/trunk@181998 bbb929c8-8fbe-4397-9dbb-9b2b20218538