fuchsia-binary-size seems inactionable

[Etienne Pierre-Doray]

Hi fuchsia devs,

TL;DR: I don't think fuchsia-binary-size is working right.

I have a CL that's blocked by fuchsia-binary-size with "Size check failed"

A few weird things that lead me to question the bot:

- The documentation suggest looking at "Read diff results" stdout, this is empty

- The diff size it complains about is the entire size of web_engine and chrome_fuchsia; looking at a normal run all I see is empty sizes:

```

"compressed": {"web_engine": 0, "cast_runner": 0, "chrome_fuchsia": 0}

```

- The debug link on the bot shows me a python stack trace

```

...

File "/b/s/w/ir/kitchen-checkout/recipe_engine/recipe_modules/step/api.py", line 716, in __call__
    return self._run_or_raise_step(
  File "/b/s/w/ir/kitchen-checkout/recipe_engine/recipe_modules/step/api.py", line 470, in _run_or_raise_step
    ret = self.step_client.run_step(step_config)
  File "/b/s/w/ir/kitchen-checkout/recipe_engine/recipe_engine/recipe_api.py", line 257, in run_step
    return self._engine.run_step(step)
  File "/b/s/w/ir/kitchen-checkout/recipe_engine/recipe_engine/internal/engine.py", line 440, in run_step
    caught = _run_step(
  File "/b/s/w/ir/kitchen-checkout/recipe_engine/recipe_engine/internal/engine.py", line 977, in _run_step
    traceback.print_stack(file=buffer)

```

- The result of the bot seems random: a diff that's seemingly a no-op made the bot go from green to red.