Enable Siso on builds

[briantting]

Enables Siso as a drop-in replacement for Ninja. Sets up Siso backend config via gclient sync and Siso toggles via GN. Deprecates reclient_cfgs.

Bug: 454374226

[github-actions]

🤖 Gemini Suggested Commit Message

---

build: Enable Siso as remote execution backend

Enable Siso as a drop-in replacement for Ninja to leverage
remote execution for Cobalt builds.

This change sets up the Siso backend configuration via gclient sync
and toggles its usage via GN arguments when remote execution is
enabled. It also deprecates and removes the legacy reclient
configuration files from cobalt/reclient_cfgs.
Bug: 454374226

---

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[gemini-code-assist]

Code Review

The pull request successfully enables Siso for Cobalt builds by integrating its configuration into the DEPS file and configure_siso.py script, and by creating a dedicated cobalt.star backend configuration. It also correctly deprecates the old reclient_cfgs by removing relevant files and configuration in gn.py. The changes are well-aligned with the pull request description and appear to be functionally correct.

[gemini-code-assist]

The container_image string is a long, critical configuration value. Extracting it into a named constant would improve readability and make it easier to locate and update if the image SHA256 ever needs to change.

    _SISO_CHROMIUM_LINUX_IMAGE = "docker://gcr.io/chops-public-images-prod/rbe/siso-chromium/linux@sha256:ef35d347f4a4a2d32b76fd908e66e96f59bf8ba7379fd5626548244c45343b2b"
    container_image = _SISO_CHROMIUM_LINUX_IMAGE