[css-layout-api] childBreakTokens attribute should be FrozenArray<ChildBreakToken>

[foolip]

https://drafts.css-houdini.org/css-layout-api/#breaking-and-fragmentation

[Exposed=LayoutWorklet]
interface BreakToken {
    readonly attribute sequence<ChildBreakToken> childBreakTokens;
    readonly attribute any data;
};

Per Web IDL, "Sequences must not be used as the type of an attribute or constant."

The fix is probably to use FrozenArray<ChildBreakToken> instead.

Found by @tidoust in w3c/webref#3 (comment).

[tabatkins]

This issue was moved to w3c/css-houdini-drafts#758