Autotest: new accessibility test to check for ChromeVox audio

Test to ensure chromevox is actually speaking on a physical device.  Uses
loopback and cras_utils to record 1 second pieces of audio and determine whether
audio is present.

Checks for audio after:
- enabling ChromeVox
- opening a new tab
- loading a webpage

BUG=None
TEST=passes on multiple devices; Fails when test does not enable Chromevox,
     or after triggering crbug.com/414966

Change-Id: I2f583c4ea59f65ebc2c44a57ae2e749e074b628e
Reviewed-on: https://chromium-review.googlesource.com/219140
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Rohit Makasana <rohitbm@chromium.org>
Commit-Queue: Katherine Threlkeld <kathrelkeld@chromium.org>
Tested-by: Katherine Threlkeld <kathrelkeld@chromium.org>
4 files changed