Background:
Pinpoint is the new performance bisection service. It has the ability to run perf bisections and perf tryjobs.

During the course of a job, it may encounter a revision that has not been built previously by the perf waterfall. Usually, these are DEPS rolls (e.g. chromium@a + v8@b) or revisions with a patch applied. So Pinpoint has a set of builders on tryserver.chromium.perf that are set up to mimic the perf waterfall builders by using the chromium_trybot recipe.

Error:
When running a build that does have a patch, the build runs green and produces all the correct artifacts.
https://build.chromium.org/p/tryserver.chromium.perf/builders/Mac%20Builder/builds/94

When running a build without a patch, it produces an AssertionError.
https://build.chromium.org/p/tryserver.chromium.perf/builders/Mac%20Builder/builds/99

Here's a CL to change the AssertionError. Let me know if I should do something else instead.
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/645835

Traceback (most recent call last):
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/run.py", line 328, in _new_run
    recipe_result = recipe_script.run(api, properties)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 98, in run
    self.run_steps, properties, self.PROPERTIES, api=api)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 627, in invoke_with_properties
    **additional_args)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 588, in _invoke_with_properties
    return callable_obj(*props, **additional_args)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/recipes/chromium_trybot.py", line 45, in RunSteps
    return api.chromium_tests.trybot_steps()
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 885, in trybot_steps
    disable_deapply_patch) = self._trybot_steps_internal()
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 930, in _trybot_steps_internal
    self.m.tryserver.set_subproject_tag(subproject_tag)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 745, in _inner
    return func(*a, **kw)
  File "/b/rr/tmp15cVp6/rw/checkout/scripts/slave/.recipe_deps/depot_tools/recipes/recipe_modules/tryserver/api.py", line 109, in set_subproject_tag
    assert self.is_tryserver
AssertionError