Rewriting WebRTC output level tests to be more reliable under load.

Rewriting WebRTC output level tests to be more reliable under load.

This greatly reduces the variance in test execution time when running
in parallel on a heavily loaded bot, in which setTimeout becomes
increasingly unpredictable. These tests were quite sensitive to load -
they run 200 consecutive setTimeout calls with short timeouts, but if
the machine is under load it can take up to 1000 ms to call back to
a 50 ms setTimeout. This means the test would take 200 * 1000 =
200 seconds to run, but it obviously times out long before then.

Running with this patch, the tests reliably stay under 10 seconds of
execution time even under the most heavy load, which is a massive
improvement to reliability. The only risk I see with this patch is that
the tests will become more resource-hungry when there are resources
available, so maybe they will starve out other sensitive tests?

BUG=472087
Committed: https://crrev.com/7fb54b10aeebc34263831fa55307cb333e67dba2
Cr-Commit-Position: refs/heads/master@{#329400}

[phoglund_chromium, 2015-05-12 09:52:24]

phoglund@chromium.org changed reviewers:
+ kjellander@chromium.org

[kjellander_chromium, 2015-05-12 12:38:47]

That's an elegant solution for this problem!
lgtm but I suggest you move some of the CL description into a review message
instead, since the different patch sets will not be available in the Git commit.

[phoglund_chromium, 2015-05-12 12:46:17]

Moving some of the patch's history here:

"I first tried writing a smarter, compensating setInterval (patch set 1), but
that didn't really help. Tests would execute well except if they were starved,
and the test execution time varied wildly between 3 seconds and 40+ seconds.
Patch set 2 removes the setTimeout calls and just enqueue the next getStats in
the getStats handler itself. It could still happen that the callback from
getStats get starved, but at least we have removed one layer of possible delays
by eliminating the setTimeout."

[phoglund_chromium, 2015-05-12 12:46:28]

The CQ bit was checked by phoglund@chromium.org

[commit-bot: I haz the power, 2015-05-12 12:47:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1132483003/60001

[commit-bot: I haz the power, 2015-05-12 13:52:38]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-05-12 13:53:28]

Patchset 4 (id:??) landed as
https://crrev.com/7fb54b10aeebc34263831fa55307cb333e67dba2
Cr-Commit-Position: refs/heads/master@{#329400}