[M73] Set page_url in CSBRR proto sent on APK downloads

Also, add a comment in the proto.
See http://b/123719599 for details.

R=drubery
TBR=vakh@chromium.org

(cherry picked from commit 497d3fb539cfcc55def4f0e83db838853c06d953)

Bug: 907280,927831
Change-Id: Idc690f25bfc3b909eead54c0184184bcf0df8a32
Reviewed-on: https://chromium-review.googlesource.com/c/1450415
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#628425}
Reviewed-on: https://chromium-review.googlesource.com/c/1452756
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#179}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
2 files changed