Enable EncryptedClientHelloQuic by default

This does *not* enable ECH by default as ECH for QUIC is additionally
conditioned on the EncryptedClientHello flag. This just makes the
existing ECH flag control both, to make managing the experiment easier.

Although we haven't implemented the recovery flow on the QUIC side,
I've confirmed the existing QUIC -> TCP fallback still works to
recover things, so that's good enough to proceed with for now.

Keeping the flag around for a little while in case of problems.

Bug: 1287248
Change-Id: Ib8cc9281a5a4e2e0533545270c317bd6211b2e86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548561


Commit-Queue: David Benjamin <davidben@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146717}