Ignore zero-height fragments during margin collapsing

1) Ignore zero-height fragments during margin collapsing step.
See https://www.w3.org/TR/CSS2/box.html#collapsing-margins
2) Change the order of expected/actual for all EXPECT_EQ in ng_block_layout_algorithm_test
3) Add more test expectations for the collapsing margin logic.

BUG=635619

Review-Url: https://codereview.chromium.org/2350603002
Cr-Commit-Position: refs/heads/master@{#419982}
7 files changed