Repro steps:

Try to see if there is an existing RenderFrameCreatedWaiter class:
1. Search for overrides of content::WebContentsObserver::RenderFrameCreated
2. See that there is quite a few.
3. Try to filter out non-test overrides.

Expected behavior: There is a way to filter out non-test XRefs

Actual behavior: "Xref Filter" has only "Exclude tests" checkbox, but there is no symmetrical "Exclude non-tests".

Maybe instead of having "Exclude foo" checkboxes, we should have checkboxes (by default all checked) for:
- Include test code
- Include product (i.e. non-test) code
- Include generated code
- Include commiteed (i.e. non-generated) code