These were removed by commit a5c4ddcf
but the assertions inevitably triggered by their removal in the
~SwAccessibleMap about "Frame map should be empty after disposing the
root frame" were left intact, which (along the total lack of any stated
reason) casts some doubt on how well thought out that change was.

Basically SwAccessibleMap had the invariant that only visible frames had
a valid SwAccessible in the mpFrameMap, and when the frames scrolled
out of view their SwAccessible was disposed.

Let's try to restore this invariant.

The dispose removal has caused crashes in the past too, see
commit 104ed86c.

Change-Id: I4b3e7264ce76a8c6e551d68f6bc231982970dfdd