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This CL addresses a Blink Accessibility invalidation gap, when a node's
aria-hidden state changes, its blink accessibility subtree must also be
invalidated so that the accessibility ignored and invisible states are
serialized to the browser process.

This is important because we rely on these states being accurate in the
browser process, and we need need to receive the state changes through
AXEventGenerator in order to send appropriate platform accessibility
events, such as structure change events (add/remove child, hide/show,
or 'children-changed').

Adding AXObjectCacheImpl::HandleAriaHiddenChangedWithCleanLayout which
handles this invalidation, and immediately handles
ChildrenChangedWithCleanLayout on the parent of the modified node,
and increments |modification_count_| to invalidate AXObject cached
values. e.g. AccessibilityIsIgnored(), and IsInertOrAriaHidden().

I needed to also modify BrowserAccessibilityWin::CanFireEvents to allow
events to process for the corner case outlined in Bug:1159660.
Because the node becoming ignored was the only child, the parent became
a leaf node, and the node removed would return false for CanFireEvents
because now it "IsChildOfLeaf()". I added a condition to check if the
ignored state changed this frame, and if so, allow events to process.
BrowserAccessibilityManagerWin filters platform events appropriately
for nodes that are ignored.

Bug: 1159660
Change-Id: I82e2368c09d4855fdb834600aed7ac61d1883c65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612365
Commit-Queue: Adam Ettenberger <Adam.Ettenberger@microsoft.com>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846165}