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Assistive software cannot readily distinguish between various
user or page actions when it receives an accessibility event.
For example, if a text change or a selection change event is
raised, the AT will not know if the text changed because the
user has typed something, if a script has written something
to the page, or if the user has pasted something.
This will affect the way that the AT might choose to
verbalize the newly added text, including choosing
not to verbalize the text at all.

On the Mac, VoiceOver needs to be told the cause of
selection change and text change events, AKA the event intents.
On Windows, the AT currently relies on monitoring the
keyboard in order to identify such intents.
On Chrome OS, we currently rely on heuristics.

This patch creates a new class, AXEventIntent, which describes the
cause of an accessibility event, e.g. "cut to the end of word".
Code in Blink should use instances of ScopedAXEventIntent to place
information about which intents are active on the stack.
This patch modifies the accessibility event dispatch mechanism
to include all active intents with each event that is sent to
the browser.

Relnotes: N/A

R=dmazzoni@chromium.org

Change-Id: I547e1c8cd980c1ec7b3a265d085317aae27c86f2
Bug: 989156
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031047Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762642}