[Eche] Send stream action go back to the Eche.

In the past, Eche used a browser back button to trigger the back action
in the stream but the back button is disabled when there's no browser
history. To enable the back button, Eche would create fake history
entries in the history stack.
With the new UX, we have our own back button so we can now directly
send the action without needing to activate the browser back button.

UX requirment: go/exo-ux-por

Bug: b/228909439
Test: Pass local build
Change-Id: I3faed666516ec5c92a0aaf8fed4ee8dd6b928aaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3587948
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Daniel Nishi <dhnishi@chromium.org>
Commit-Queue: Paulz Chen <paulzchen@google.com>
Cr-Commit-Position: refs/heads/main@{#994478}