Protect UpdateRTT from negative send deltas

Protect UpdateRTT from negative send deltas

Prevent negative send_deltas from being incorporated into the various
RTT calculations.  AFAIK, it's not happening now, but it doesn't hurt to
be vigilant against future bugs.

If we're getting such RTTs, it's due to some pathological error that
we'd like to know about.

Added a unittest for these corner cases.

Low-impact. Don't use negative samples in QUIC rtt calculation.

Merge internal change: 78580114

R=rch@chromium.org

[ramant (doing other things), 2014-10-29 02:01:31]

Hi Ryan,
  Our version of ScopedMockLog and LOG(...) are different. Made changes to make
UpdateRttWithBadSendDeltas work in chromium build tree (on linux). Will see if
it works on other platforms or not.

[Ryan Hamilton, 2014-10-29 03:00:03]

On 2014/10/29 02:01:31, ramant wrote:
> Hi Ryan,
>   Our version of ScopedMockLog and LOG(...) are different. Made changes to
make
> UpdateRttWithBadSendDeltas work in chromium build tree (on linux). Will see if
> it works on other platforms or not.

If you're unable to make this test work, don't sweat it. I don't think this test
is that important.

[Ryan Hamilton, 2014-10-29 03:00:09]

lgtm