Add ClearanceOffset to LayoutNG Constraint space.

It's used for the case when fragment can position itself but needs to
know the current clearance line.

In particular this change fixes the use case that has blocks with
clearances, margins and intruding floats
https://software.hixie.ch/utilities/js/live-dom-viewer/saved/4847

BUG=635619
TEST=PositionBlocksWithClearanceAndIntrudingFloats

Review-Url: https://codereview.chromium.org/2711803007
Cr-Commit-Position: refs/heads/master@{#453036}
9 files changed