Add histogram to record fallback result of WebSocket

There are some histograms that record WebSocket handshake results.
However none of them record whether a WebSocket handshake got involved
with HTTP/1.1 fallback. Add a histogram that records whether
WebSocket handshakes get involved with fallback.

Bug: 1455328
Change-Id: I7d352f5c11948398cd6d1e8081cc39017c8b4234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4618051
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158739}