Remove tree_status experimental builders from completed_builds set

SlaveStatus.completed_builds set keeps track of completed and importat
builds. If a build was important and got added to the completed_builds
before, but is marked as experimental in tree status in current run,
should take it out from the completed_builds set.

BUG=chromium:758247
TEST=unit_tests

Change-Id: I9dacfd9d0c1bd4e023868453333c0aa0a756fb0d
Reviewed-on: https://chromium-review.googlesource.com/633888
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Ningning Xia <nxia@chromium.org>
2 files changed