Add argument use_poll=True to asyncore.loop() calls in perf_snapshot.py.

Add argument use_poll=True to asyncore.loop() calls in perf_snapshot.py.

This change causes asyncore.loop() to internally use poll() instead of
select(), which may prevent the "bad file descriptor" exception from being
thrown on the Chrome Endure tests.

Also enhanced an assertion error message in perf_endure.py to give more
context if the assertion is violated.

BUG=None
TEST=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=119983

[dennis_jeffrey, 2012-01-31 23:04:37]

Before having the Chrome Endure tests tolerate asyncore.loop() exceptions, I'd
like to try this change and see if it prevents the "bad file descriptor"
exception from being thrown in calls to asyncore.loop().

[Nirnimesh, 2012-01-31 23:09:19]

LGTM