Use failure.tree field provided by the builder_alerts generator

Depends on https://codereview.chromium.org/508873005/.

BUG=406969
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/511253004

git-svn-id: svn://svn.chromium.org/blink/trunk@181652 bbb929c8-8fbe-4397-9dbb-9b2b20218538