This changes the `detail` of the ViewShowing events in an incompatible way.
There were only a few places it was used in mozilla-central, which I fixed. I
searched add-ons in DXR and didn't find any that used the `detail` member of
this event, but I had to resort to a random sampling, so it's possible that
some do exist.
MozReview-Commit-ID: CYzGw6KH7uI
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amend-0
MozReview-Commit-ID: EZfo1k7YhSx
efcf532168