Fragment that establishes new BFC knows it's BFC offset in parent space

List of changes:
- If a fragment establishes new BFC then we can collapse its margin and
  assume that it's BFC offset is known. We can't use
  fragment->BfcOffset() in this case because each BFC has it's own BFC
  coordinates.
- Revert lines that changed the usage of BFC offset to 'unified' offset
- s/CalculateRelativeOffset/CalculateLogicalOffset/g
- other minor clean up

BUG=635619
TESTS=NGBlockLayoutAlgorithmTest::CollapsingMarginsCase5

Review-Url: https://codereview.chromium.org/2679343002
Cr-Commit-Position: refs/heads/master@{#449223}
6 files changed