Create LayoutNG virtual test suite for fast/block/margin-collapse path.

This creates a LayoutNG virtual test suite for fast/block/margin-collapse path and switches to use "Skip" instead of "Failure" to mark tests that LayoutNG doesn't support at this moment.

BUG=635619

Review-Url: https://codereview.chromium.org/2396583002
Cr-Commit-Position: refs/heads/master@{#422923}
3 files changed