Send `Sec-Fetch-User` only for user-activated, navigational requests.

As per the conversation in mikewest/sec-metadata#23 and
mikewest/sec-metadata#19, this patch drops the `Sec-Fetch-User` header
for non-navigational requests, and for navigational requests that are
not user-activated.

Bug: 947444
Change-Id: Ica4846bda6ccf4e8bce1323803954f4fef9c18a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1545871
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646086}
42 files changed