Fix automation event listeners in Select-to-speak

Fixes a regression caused by http://crrev.com/c/2424854 ("Refactor
chrome.automation.EventType"), it could have manifested as
select-to-speak not updating when the page scrolls.

See http://crrev.com/c/2450663 for more context.

Bug: 1110367, 1135248
Change-Id: I12e62a18093a0fc2837ec0be312d4f7261c5dc42
AX-Relnotes: none because this just fixes a regression
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454633
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814738}