Add CSS parser support for the snap-height property

This patch adds CSS parser support for the snap-height property, behind
the CSSSnapSize runtime flag.

Supporting layout will be in following patches.

A test was imported from csswg-test and was marked as failure. This
patch makes the test pass.

Spec: https://drafts.csswg.org/css-snap-size/
Dashboard: https://www.chromestatus.com/features/5734273533345792

BUG=586413
TEST=imported/csswg-test/css-snap-size-1/snap-height-parsing-001.html

Review URL: https://codereview.chromium.org/1696373003

Cr-Commit-Position: refs/heads/master@{#376051}
14 files changed