[Feed] Record age difference between Feed and Host.

Bug: 936155
Change-Id: I13d30f3a8d3893e2e27b696797f09cc5c5643495
Reviewed-on: https://chromium-review.googlesource.com/c/1490733
Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Gang Wu <gangwu@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636055}
2 files changed