Forward accessibility events to the automation extension process.
See bug for context. This patch adds code to
keep track of extensions using the automation API
that want to subscribe to accessibility events and
implements an IPC to forward the accessibility events.
The next CL will complete the implementation
based on this new IPC and delete the code that
forwards accessibility events using the extension
event router.
BUG=495323
Review URL: https://codereview.chromium.org/
1151523009
Cr-Commit-Position: refs/heads/master@{#333616}