Ignore disconnect signals when a QUIC connection migration is happening on the default network

Suppose that both wifi and mobile networks are active and the default
network is wifi. When wifi becomes unavailable, there could be two
signals from NetworkChangeNotifier via NetworkObserver to trigger
a QUIC connection migration:
- OnNetworkDisconnected() to signal wifi becomes inactive
- OnNetworkMadeDefault() to signal mobile becomes the default network

The former doesn't require proving, but the latter requires proving.

The order of these signals is not guaranteed. Before this CL,
QuicChromiumClientSession didn't handle the case where
OnNetworkMadeDefault() comes first and the OnNetworkDisconnected()
comes next. There was no valid connection IDs until proving finished,
but QuicChromiumClientSession::OnNetworkDisconnectedV2() tried to
migrate immediately, resulting in a migration failure.

To avoid the problem, ignore disconnect signals when a migration is
happening on the default network.

The existing QuicConnectionMigrationTest.Basic test will cover this
change once it starts running on bots.

Bug: 1509458
Change-Id: I9b348ad82ef004800356f5b63c1f702c1d09ccca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5138419
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Cr-Commit-Position: refs/heads/main@{#1242768}