Add alerts for possibly hung buildslaves
Add alerts for possibly hung buildslaves

If any step runs for over an hour, trigger an alert. Exceptions for
Valgrind and Chrome canaries.

BUG=skia:3991

Review URL: https://codereview.chromium.org/1213233010

google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot