Add a missing PerformanceTiming readonly attribute name

[JosephPecoraro]

Any PerformanceTiming interface readonly attribute name should cause a SyntaxError when used by performance.mark in Window. Although this attribute is specially optional, it is a readonly attribute in the interface, so it should be tested.

---

Spec for performance.mark:

1. If the global object is a Window object and markName uses the same name as a read only attribute in the PerformanceTiming interface [NAVIGATION-TIMING-2], throw a SyntaxError.

Spec for PerformanceTiming:

interface PerformanceTiming {
    ...
    readonly attribute unsigned long long secureConnectionStart;
}

Spec for secureConnectionStart:

secureConnectionStart
This attribute is optional. User agents that don't have this attribute available must set it as undefined. [...]

---

Firefox does not appear to support secureConnectionStart yet. So adding this attributes causes Firefox to fail a few tests.

An alternative solution would be to only add "secureConnectionStart" to this list if it is supported by the browser, but I'm not sure that matters given the spec text above. Any read only attribute should throw a SyntaxError. I don't think supported or not matters.

[wpt-pr-bot]

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These tests will be available shortly on w3c-test.org.

w3c-test:do-not-mirror (What's this?)

[JosephPecoraro]

Hmm, the existing test_user_timing_mark_with_name_of_navigation_timing_optional_attribute.html does attempt to special case this optional attribute.

I'll leave it up to reviewers to suggest the path forward here. Should we special case this attribute in all tests or not.

[igrigorik]

I think this is blocked on w3c/user-timing#22.

[plehegar]

The attribute was added in #5247, so feel free to close this PR.

[igrigorik]

sgtm, thanks!