Fix incorrectly calculated content_size for fragments that create new FC

This patch adds MarginStrut.BlockEndSum to the fragment's content size
if the fragment establishes a new formatting context.

The change is tested with layout_ng virtual test suite located in fast/block/basic.

BUG=635619

Review-Url: https://codereview.chromium.org/2389823003
Cr-Commit-Position: refs/heads/master@{#422878}
6 files changed