cros_utils/buildbot_utils: leverage `cros buildresult`

Originally it sends a http request to some magic URL and parse the
contents. Now there is a `cros buildresult` available for use.

BUG=chromium:807029
TEST=cros_utils/buildbot_utils_unittest.py
     buildbot_test_toolchains.py

Change-Id: I33441859afd675c1a6b70d3d1f4e2348d54cbd77
Reviewed-on: https://chromium-review.googlesource.com/954052
Commit-Ready: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
4 files changed