overlord: eliminate the use of browser_id

In the previous commit we allow clients to send terminal control message
to the frontend browser, which passes terminal session ID to frontend.
We can now use this session ID as subscripbtion channel without having
an extra browser ID.

TEST=manually test upload and download function

Change-Id: Id59318086d496916ec8c3f89d97558318816efbc
Reviewed-on: https://chromium-review.googlesource.com/291384
Reviewed-by: Hsu Wei-Cheng <mojahsu@chromium.org>
Commit-Queue: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
5 files changed