Check CSP synchronously for string timer handlers

[zcorpan]

Check CSP synchronously for string timer handlers

As in implementations, a blocked call now returns 0 without reporting an exception, and the check is done only once per call.

Fixes #13018.

• At least two implementers are interested (and none opposed):
  • Chromium, Gecko, and WebKit already behave this way
• Tests are written and can be reviewed and commented upon at:
  • Test that timers check CSP synchronously for string handlers web-platform-tests/wpt#63204
• Implementation bugs are filed:
  • Chromium: N/A (passes)
  • Gecko: N/A (passes)
  • WebKit: N/A (passes)
  • Deno: N/A (no CSP)
  • Node.js: N/A (no CSP)
• Corresponding HTML AAM & ARIA in HTML issues & PRs: 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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