Contains multiple improvements related to bbagent.

R=gbeaty@chromium.org, yiwzhang@google.com

Bug: 1015181, 1087651
Change-Id: I1ee63fe2e3f0d2a717725f3c01e6159af635dae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2220867Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>