Add AXPlatformNode ObserverList so that clients know when ax mode changes.

Bug: 703369
Change-Id: Ia1d64a9b99b952f6391eda39ce98a2ae6ad6fa20
Reviewed-on: https://chromium-review.googlesource.com/601541
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Doug Turner <dougt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494578}
10 files changed