The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/88aecf78de345710017519c764ce359a4fbf3b1b

commit 88aecf78de345710017519c764ce359a4fbf3b1b
Author: Randy Rossi <rmrossi@chromium.org>
Date: Thu Aug 23 17:47:48 2018

Fix chromevox ready message on chromecast

This CL removes the platform check for windows/linux/mac
when deciding when to announce the startup message.  The message
should be spoken for all platforms. This script is not executed
on Android anymore so there is no longer any reason to prevent the
message from being spoken on that platform.  For ChromeOS, however,
the existing onIntroduceChromeVox listener is used to announce
startup.  The criteria therefore changed to !isChromeOS.

      Installed on Chromecast and verified message is spoken to fix the
      bug.

Bug: 875935
Test: Installed on ChromeOS and verified message still spoken
Change-Id: I078ee4aaab6ed3cedabcc43edbb6660aa9c7523e
Reviewed-on: https://chromium-review.googlesource.com/1181931
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585538}
[modify] https://crrev.com/88aecf78de345710017519c764ce359a4fbf3b1b/chrome/browser/resources/chromeos/chromevox/chromevox/background/background.js