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This is a follow up change of CL:1667787, which introduced
AutofillPopupControllerImpl::FireControlsChangedEvent for autofill
popup accessibility.

On Mac and potentially other platforms, when accessibility is
enabled, upon invoking autofill popup and FireControlsChangedEvent
we end up dereferencing a nullptr of AxPlatformNode due to
Mac does not have a complete implementation of AxPlatformNode yet.

This CL fixes the above by adding a check for AxPlatformNode in
FireControlsChangedEvent.

Changes:
 1. Introduced a check for invalid AxPlatformNode in
    AutofillPopupControllerImpl::FireControlsChangedEvent to return
    early from firing event.
 2. Added associated unit tests for FireControlsChangedEvent.
 3. Exposed GetWebContentsPopupControllerAxId virtual to
    AutofillPopupDelegate..

Bug: 986587
Change-Id: I9a6ed49330a9ea9d7d4e5483c3ee06d5675919ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714297
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686461}