Withhold origin trial features from API Reference generation

The API reference displays items that are in origin trials without indicating them as such. This has a number of problems.

* The existence of an origin trial is no guarantee of shipping. In eight years, I've seen features that were shipped with a different API surface, I've seen features that went through a second origin trial with a different design, and I've seen features that did not ship at all.
* Because this feature is only available in an origin trial, extension developers can't count on its availability. To even use it, host sites must have opt-in to the origin trial. In the most optimistic case, this feature is only available to 1 web site in every 1.5 million.
* Our documentation gives no indication that this is experimental, much less how I could test it if I wanted to.
* We do not publicize origin trials that affect extensions and provide no general guidance for how an extension developer can participate in an origin trial.
* Do we even want extension developer participating in origin trials?

For example, so that extensions can handle [Back/Forward (sic) caching](https://chromestatus.com/features#back%20forward%20cache) a few things have been added to extension APIs. One of them is called [`FrameType`](https://developer.chrome.com/docs/extensions/reference/extensionTypes/#type-FrameType). One of the values of `FrameType` is `"fenced_frame"`. But [Fenced Frame ](https://chromestatus.com/feature/5699388062040064) is in an origin trial that finished by the time Chrome shipped 108 on November 29.

**This issue is a request to either hide or correctly label items that are in origin trials.** I favor hiding since labeling such items requires additional documentation and publicity that may or may not be useful to our developers.

Regardless of which option we chose, the first step will be making the conversion process aware of origin trials.