Apply the percentage height quirk for position: sticky

[zcorpan]

Fixes #83.

• At least two implementers are interested (and none opposed):
  • Already matches Gecko/WebKit/Chromium
• Tests are written and can be reviewed and commented upon at:
  • Quirks: Apply percentage height quirk to position: sticky web-platform-tests/wpt#54266
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: N/A
  • WebKit: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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