Reverts "Remove Cocoon from `dev/devicelab`, keeping Skia perf stats upload. (#165749)" (#165754)

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Reverts: flutter/flutter#165749
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Initiated by: matanlurey
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Reason for reverting: Still passing command-line arguments from recipes
that have no effect but cause the runner to crash.
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Original PR Author: matanlurey
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Reviewed By: {jtmcdole}
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This change reverts the following previous change:
Partial re-land of https://github.com/flutter/flutter/pull/165628:

- Fixes the mistake that the `Cocoon` class did things that well, were
not specific to Cocoon.
- Renamed to `MetricsResultWriter`, as that is all it does now.

---

Closes https://github.com/flutter/flutter/issues/165618.

The `devicelab/bin/test_runner.dart upload-metrics` command use to have
_two_ responsibilities:

- Well, upload test **metrics** (benchmarks) to Skia Perf (it still does
that)
- Upload test **status** to Cocoon (it did until
https://github.com/flutter/flutter/pull/165614)

As https://github.com/flutter/flutter/pull/165614 proved, this API
predated the current LUCI setup, where Cocoon itself receives task
status updates from LUCI, and it turns out this entire time, DeviceLab
was making (at best) NOP calls, and at worst, causing crashes and
corrupt data (https://github.com/flutter/flutter/issues/165610

).
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Co-authored-by: auto-submit[bot] <flutter-engprod-team@google.com>