Intent to Implement and Ship: CSS overscroll-behavior-{block,inline}

[Sean Voisen]

Summary: The overscroll-behavior-{block,inline} CSS properties are
flow-relative versions of the already-supported overscroll-behavior-{x,y}
longhand properties.

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1453472

Standard:
https://drafts.csswg.org/css-overscroll/#overscroll-behavior-longhands-logical

Discussion: https://github.com/w3c/csswg-drafts/issues/2473

Platform coverage: All platforms

Estimated or target release: Firefox 73

Preference behind which this will be implemented: These are behind the same
preference that is already enabled for the existing overscroll properties,
layout.css.overscroll-behavior.enabled.

DevTools bug: N/A, devtools support is included in these patches.

Other browser support:
https://wpt.fyi/results/css/css-overscroll-behavior/overscroll-behavior-logical.html
<https://wpt.fyi/results/css/css-logical/logical-box-border-shorthands.html>

web-platform-tests:
http://w3c-test.org/css/css-overscroll-behavior/overscroll-behavior-logical.html

Secure Contexts: Enabled for both secure and insecure contexts.

Sandboxed iframes: This feature will be enabled by default in sandboxed
iframes.