Remove all:unset from select button

[josepharhar]

all:unset is not very performant and was just a convenient way to remove appearance:auto UA styles. We can use appearance:base instead, which should achieve the same goal without reseting every property.

Fixes #12121

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • TODO
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs: not needed
• MDN issue is filed: not needed
• The top of this comment includes a clear commit message to use.

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

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