[Spec] Fix navigation from the default constructor not installing a fenced frame config.

[blu25]

The existing spec results in a fenced frame navigating directly to the url stored in a FencedFrameConfig IDL object when navigating to a config built with the FencedFrameConfig(url) default constructor. This results in a fenced frame config and fenced frame config instance not being created for the fenced frame, cutting off the internal document from window.fence.

This PR fixes that in the following ways:

• Modifies the FencedFrameConfig(url) constructor to also create a fenced frame config and add it to the navigable's fenced frame config mapping.
• Unconditionally passes in the FencedFrameConfig's urn when navigating the internal document, ensuring that the navigation will find the associated fenced frame config, and instantiate a fenced frame config instance as expected.
• Removes the url field from the FencedFrameConfig in favor of using the mapped url in the fenced frame config/fenced frame config instance.
• Adds a method to store a new FencedFrameConfig directly in the finalized mapping without first putting it in the pending mapping.

This PR also adds a default fenced frame effective sandboxing flags constant, which is needed when building the fenced frame config in the FencedFrameConfing(url) constructor.

See issue: https://github.com/WICG/fenced-frame/issues/178

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