'Sec-Fetch-Dest' should return correct values for <object> and <embed>.

Rather than blindly assigning "nested-document", we ought to examine the
state of the frame's owner to determine whether it might potentially end
up executing as a plugin, and report the value in the header accordingly.

Bug: 860510
Change-Id: I78291d8209382fbd325f4d8e72b24c6444fe25b6
Reviewed-on: https://chromium-review.googlesource.com/c/1432955
Auto-Submit: Mike West <mkwst@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626155}