Adding flag for DiscoFeed endpoint.

Change-Id: I02969906417cb7311778c444be149d3ddcc91084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3866056


Reviewed-by: Dan H <harringtond@chromium.org>
Auto-Submit: Jonathan Freed <freedjm@google.com>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041595}