Use Layout Opportunity Iterator to position new FC blocks.

This patch fixes the use case when we need to use Layout Opportunity
Iterator to position a block that establishes a new formatting context.

BUG=635619
TEST=NGBlockLayoutAlgorithmTest.NewFcBlockWithAdjoiningFloatCollapsesMargins

Review-Url: https://codereview.chromium.org/2816933003
Cr-Commit-Position: refs/heads/master@{#465899}
14 files changed