Introduce PageLoad UMAs for prefetched alternative SXG

This CL introduces PageLoad.Clients.SignedExchange.AltSubSXG.* UMAs
which are recorded when prefetched alternative subresource signed exchange
is sent to the renderer process.

Bug: 935267
Change-Id: Ifb74ce9c84e67eac9bc930d13ae86f719341500a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697050
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676797}
7 files changed