[LayoutNG] Fix IsEmptyBlock check within block_layout_algorithm.

Previoiusly we were just checking fragment.BlockSize() which was error
prone as:
<div style="height: 0px;">text</div>
... would have been considered an empty block.

Now the check uses the presence of the BfcOffset on the child's layout
result. If it isn't present, the child must be empty.

BUG=635619

Review-Url: https://codereview.chromium.org/2982643002
Cr-Commit-Position: refs/heads/master@{#486440}
2 files changed