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Google drive expand/collapse state is not being announced due to
excessive HIDE/SHOW/REORDER events flooding NVDA, and NVDA end up
discarding most events.

For example, if we remove aria-hidden="true" on an ancestor node, which
would cause that many of its descendants to remove their IGNORED
states which triggering too many EVENT_OBJECT_SHOW and causes NVDA
to drop the events. In reality, we only want EVENT_OBJECT_SHOW to be
generated on the ancestor node where aria-hidden="true" is removed, so
not to flood and confuse NVDA with excessive events.

This change fixes the above issue by firing EVENT_OBJECT_SHOW on the
root should the nodes of the entire subtree remove their IGNORED states.

https://crrev.com/c/2309450 implemented suppressing excessive HIDE
events, of which this CL is based on.
Suppressing excessive REORDER events will be addressed in follow up
patches.

AX-RelNotes: NVDA can now announce Google drive "My Drive" expand/
collapse state.

Bug: 1019420
Change-Id: Icecf5d998ea666db4bf21a56f8b8b0b9a1e4aaf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337404
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Adam Ettenberger <Adam.Ettenberger@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#802054}