This revision changes the logic for creation and updating of accessibles
corresponding to img elements such that alt="" (without click listeners or any
other aria attribute that forces an accessible) will effectively remove that
img's accessible from the accessibility tree. Next, this revision removes the
concept of eNoNameOnPurpose from accessible name handling, since it's now
unnecessary now that alt="" means "don't create an accessible" in most cases.
This revision also adds tests to verify the functionality and updates existing tests.
Differential Revision: https://phabricator.services.mozilla.com/D160706
8c8a637b75