Async tail might be long. On frontend side we use only top frame so
we can report tail using id.

R=dgozman@chromium.org

Bug: chromium:873865
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie9e6b5c4c000cc6bedce2d5fec9f3fa22ea21768
Reviewed-on: https://chromium-review.googlesource.com/c/1286959
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56784}