Not sending QUIC packets if the send alarm is set. Behind FLAGS_respect_send_alarm

Not sending QUIC packets if the send alarm is set. Behind FLAGS_respect_send_alarm

This fixes a subtle bug where we weren't always respecting priorities when writing.

I posit that if we are send limited by the sent packet manager, we expect to
resume with OnCanWrite, and resume the highest priority session.  Unfortunately
right now that's not the case.  When the highest priority session gets blocked
by the sent packet manager it schedules the alarm. However it's perfectly
possible that before the alarm fires, we get an incoming request, process it,
that stream checks to see if it can write, and when CanWrite checks now(), we
are past the time when the alarm should have fired (due to doing a lot of work,
busy machine, what have you) and write data for the random stream.

Merge internal change: 107380116

R=rch@chromium.org
BUG=

[Ryan Hamilton, 2015-11-11 17:56:23]

https://codereview.chromium.org/1439653002/diff/1/net/quic/quic_flags.cc
File net/quic/quic_flags.cc (right):

https://codereview.chromium.org/1439653002/diff/1/net/quic/quic_flags.cc#newc...
net/quic/quic_flags.cc:124: bool FLAGS_respect_send_alarm = true;
Ian mentioned this morning that there might be a bug in this flag, so lets' set
it to false.

[rjshade, 2015-11-11 17:57:37]

On 2015/11/11 17:56:23, Ryan Hamilton wrote:
> https://codereview.chromium.org/1439653002/diff/1/net/quic/quic_flags.cc
> File net/quic/quic_flags.cc (right):
> 
>
https://codereview.chromium.org/1439653002/diff/1/net/quic/quic_flags.cc#newc...
> net/quic/quic_flags.cc:124: bool FLAGS_respect_send_alarm = true;
> Ian mentioned this morning that there might be a bug in this flag, so lets'
set
> it to false.

Done.

[Ryan Hamilton, 2015-11-11 22:13:52]

lgtm