SystemTimeZoneIdentifier should continue returning a primary time zone identifier

[anba]

From https://w3c.github.io/webdriver-bidi/#command-emulation-setTimezoneOverride:

If IsTimeZoneOffsetString(emulated timezone) returns false and AvailableNamedTimeZoneIdentifiers does not contain emulated timezone, return error with error code invalid argument.

Two issues:

1. AvailableNamedTimeZoneIdentifiers returns a List of Records, so it's not clear that contain is correctly used.
2. ECMA-262 SystemTimeZoneIdentifier is specified to return a primary time zone identifier. The WebDriver BiDi override shouldn't violate this requirement.

See also ECMA-402's GetAvailableNamedTimeZoneIdentifier and AvailablePrimaryTimeZoneIdentifiers.

• If the time zone override is expected to be called with a primary time zone identifier (in the correct case), then AvailablePrimaryTimeZoneIdentifiers can be used. For example "America/New_York" will get accepted, but not "america/NEW_YORK" (wrong case) or "EST5EDT" (time zone link to "America/New_York".)
• If "america/NEW_YORK" and "EST5EDT" should be accepted as time zone overrides, then GetAvailableNamedTimeZoneIdentifier can be used. For example:

1. If IsTimeZoneOffsetString(emulated timezone) returns false:
   1. Let timeZoneIdentifierRecord be GetAvailableNamedTimeZoneIdentifier(emulated timezone).
   2. If timeZoneIdentifierRecord is empty, return error with error code invalid argument.
   3. Set emulated timezone to timeZoneIdentifierRecord.[[PrimaryIdentifier]].

(Not sure if that's the correct syntax for W3C specs, but should be close enough.)