Use new earcons in ChromeVox Next.
Add a new Earcon class that generates sounds using
the new cvox2 EarconEngine. Swap it in dynamically
when we're in a ChromeVox Next mode. Add support for
testing earcons to MockFeedback and write an initial
test that all of the correct earcons are outputted
for controls.
BUG=
491243
Committed:
https://crrev.com/6f3dbc166c172aa3d78e06b181bdb5fb5b09d774
Cr-Commit-Position: refs/heads/master@{#354366}