AbsolutePositioning utilities with tests

These are the routines that implement absolute positioning specification.

ComputeAbsoluteHorizontal and ComputeAbsoluteVertical implement
the spec. They will be used by block_algorithm to position abspos elements.

BUG=635619

Review-Url: https://codereview.chromium.org/2497863002
Cr-Commit-Position: refs/heads/master@{#433402}
6 files changed