[spdy] Add broken connection detection API to SpdySession

Allow to enable continuous connection status monitoring for spdy
sessions. This mechanism is built on top of HTTP2 PING frames.

When enabled, a PING frame will be sent every T seconds (requestor
defined), if the other end of the connection doesn't reply the
connection is drained (shares logic with the preface PING mechanism).

This is extremely useful for long lasting connection where the
communication mostly happens from server to client (e.g., notification
pushing). In this case, if the network path between client and server
becomes bad, only the "sender" (i.e., the server) will notice, while the
"receiver" (i.e., the client) will simply think that the other end isn't
sending data.
This happens because, by default, TCP doesn't use a timeout and/or
keep-alive mechanism (they can be enabled, but we don't do that on
mobile platforms to avoid constantly waking up the radio) and will only
close a connection if it ends up retransmitting a frame too many times.
So, the server side will close the connection and send a TCP reset to
the other end, which will never be received due to the network path
being broken.

For mobile platforms this could possibly cause the radio status to be
woken up more frequently, possibly causing a regression in devices
battery life. On Android, we avoid that by listening for network active
type of events and sending the PING frames only when the radio is
already in full power mode.

Bug: 1183119, b:189096305
Change-Id: I63fb64f7d63dd756a3364fb7f33dd104f96c3035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3048165
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Stefano Duo <stefanoduo@google.com>
Cr-Commit-Position: refs/heads/main@{#960036}