Reset constraint space's BFC offset if block creates a new FC

List of changes:
1) Resolved TODO in constraint_space_builder and actually
   reset BFC offset if block creates a new FC.
2) Added InlineBlockStartOffset to NGBoxStrut
   That helped to simplify CalculateLogicalOffsetForOpportunity
3) Refactored NGFloatingObject
   remove space and original_parent_space
   use (origin|from)_offset instead
4) Moved PositionPendingFloats to ng_float_utils

BUG=635619

Review-Url: https://codereview.chromium.org/2790283003
Cr-Commit-Position: refs/heads/master@{#462156}
8 files changed