Split out QuicAlarm creation from QuicConnectionHelper to new QuicAlarmFactory. No behavior change,…

Split out QuicAlarm creation from QuicConnectionHelper to new QuicAlarmFactory. No behavior change, not protected.

This is in preparation for gRPC-over-QUIC to use a separate
QuicAlarmFactory for each GrpcQuicConnection. Currently there exists a
single Helper (and after this CL a single AlarmFactory) per Dispatcher,
and this is used to create alarms for all Connections.

gRPC-over-QUIC needs to add per-connection locking to connection alarms
before firing them, and a followup CL will create a different
AlarmFactory for each gRPC-over-QUIC connection, each with its own
mutex (rather than sharing one mutex across the alarms of connections
which is obviously terrible).

Merge internal change: 119778209

BUG=

[Ryan Hamilton, 2016-04-21 19:40:36]

rch@chromium.org changed reviewers:
+ rtenneti@chromium.org

[ramant (doing other things), 2016-04-21 21:38:58]

lgtm