[Spec] window.top can reach past a fenced frame boundary.

[blu25]

The definition of top-level traversable was modified to always get the unfenced top-level traversable. That is, calling this algorithm from within a fenced frame tree will break past the fenced frame boundary and get the outermost traversable instead.

This change has the side effect of breaking the intended fenced functionality of window.top. That algorithm returns the active WindowProxy of the top-level traversable. However, since that algorithm is unfenced, window.top is now able to reach past a fenced frame boundary and get a frame outside of a fenced frame tree, which is something we do not want the web platform to be able to do.

To fix this, we should:

1. Introduce fenced/unfenced variants of top-level traversable.
2. Have the window.top algorithm call the fenced version of top-level traversable.
3. Audit other instances where top-level traversable is called to see if it should be fenced or not, and fix those as well if necessary.
4. Based on the audit, determine if top-level traversable should be fenced or unfenced by default.