Never fires an event on a destroyed BrowserAccessibility object

Also, prevents the firing of any platform events on descendants of leaf nodes.
Such nodes are never exposed to platform APIs, but this patch makes sure that events cannot be accidentally dispatched on them.
R=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I3cd8391ef8a687d70649167579d094f449008e4e
Bug: 934529
Reviewed-on: https://chromium-review.googlesource.com/c/1483254
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634784}
3 files changed