The tryserver.require_is_tryserver method will create an infra-failing
step that raises an exception if there is no CL associated with the
build. In the case of an LED task, it will only fail so as not to cause
the removal of the builder cache in the case of a minor user error.

Change-Id: I05d3a86cc727577f067fb3eae49e46398345f672
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3413971
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>