Add a hang monitor for the audio thread.

Add a hang monitor for the audio thread.

We've seen a bunch of reports in the field that the audio thread is
getting hung for significant (perhaps indefinite) amounts of time;
to help debug these reports, add a hang monitor which will crash the
browser process and ensure we get a thread dump of what's going on.

The hang timer is set extremely conservatively right now, at 1 minute,
which is insanely long for audio tasks. I'll monitor crash reports and
adjust it as necessary to find culprits.

The hang monitor uses the browser I/O thread for hang checks, which is
similar to what other hung task monitors seem to do. To do this I had
to move AudioManager and MidiManager creation to after the browser
threads have been spun up. Which shouldn't be an issue...

BUG=422522, 469827
TEST=hang time exceeded ==> crash.

Review URL: https://codereview.chromium.org/1036703002

Cr-Commit-Position: refs/heads/master@{#322276}