Introduce PageLoad.Clients.CachedSignedExchange.* UMAs

Bug: 863305
Change-Id: Ib2656fe568543688d87193c5689eecfad640b5b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573301
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652157}
9 files changed