[WebID] Add WebID to Chrome flags

This removes WebID from experimental web platform features and adds a
new flag for it. This is because the HTTP filtering mechanism added in
r859333 is very intrusive during federated logins and should not affect
everyone who is running with that flag enabled.

Separately, this also tweaks the HTTP filtering mechanism so that it
only triggers on top-level frame navigations.

Change-Id: I9fe624d5e6a969fd397a2a8de5083d5d568fb37c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2733349
Reviewed-by: Sam Goto <goto@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859657}