Add LayoutOpportunityIterator method to NGConstraintSpace

LayoutOpportunityIterator method returns a default layout opportunity
iterator that starts from NGConstraintSpace::BfcOffset. It's invalidated
when a new exclusion is added.
Usage example: NGInlineBuilder needs to iterate over available
opportunities.

BUG=635619

Review-Url: https://codereview.chromium.org/2734923002
Cr-Commit-Position: refs/heads/master@{#455220}
4 files changed