[perf] More patience for PerfClusteringSlow alerts.
[perf] More patience for PerfClusteringSlow alerts.

Change-Id: I0642b7253fdeaa8dafbef6ef93af085bc4d17e35
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/273096
Reviewed-by: Weston Tracey <e7e27cb74e46abd7b999223aa4f27901ab7e58c6@google.com>

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