Change NGLayoutOpportunityIterator API to work with list of exclusions

This updates NGLayoutOpportunityIterator API so it starts accepting
the list of exclusions instead of constraint space.

We need this API to implement the logic that will search for layout
opportunities to place a BFC block while we have some placement
pending floats.

BUG=635619

Review-Url: https://codereview.chromium.org/2812983002
Cr-Commit-Position: refs/heads/master@{#464746}
5 files changed