Description of Change

Adds a manually-triggered (workflow_dispatch) pipeline that builds PGO-instrumented Electron for every published platform/arch combo, collects optimization profiles by running benchmark and app-style workloads against each build, and publishes merged profiles to https://dev-cdn.electronjs.org/pgo/.

Why Electron needs its own profiles

Electron release builds apply Chrome's published PGO profile (chrome_pgo_phase = 2). PGO profiles match functions by symbol name + control-flow hash, so every function that differs from Chrome — all of Node.js, patched Chromium code, V8 built with Node's flags, and the shell/ code itself — silently gets no optimization guidance and is laid out as cold. The same applies to V8's builtins profile: Chrome's published profiles reject Electron's promise/async builtins (RunMicrotasks, AsyncFunctionAwait, FulfillPromise, …) because Node's integration changes their codegen.

Measured impact of replacing Chrome's profile with an Electron-collected one (Linux x64, otherwise identical builds):

What the pipeline does

8x instrumented builds          1x instrumented d8
(chrome_pgo_phase = 1)          (v8_enable_builtins_profiling)
        |                                |
        v                                v
8x profile collection            JetStream 2 -> get_hints.py
(benchmarks + app workloads)             |
        |                                |
        v                                v
merge (llvm-profdata)            electron-v8-x64.profile
        |                                |
        +----------------+---------------+
                         v
     OIDC upload -> dev-cdn.electronjs.org/pgo/

• Platforms: linux x64/arm/arm64, win x64/x86/arm64, macos x64/arm64 — each combo collects on hardware/containers that can run its own binaries (matching how Chrome generates per-arch profiles)
• Collection workloads: Speedometer 3 (×3), JetStream 2, MotionMark — the same set Chromium's PGO recipe uses — plus Electron-specific phases that browser benchmarks can't cover: main-process Node.js (Buffer/crypto/fs/JSON), contextBridge + ipcRenderer.invoke marshaling, and renderer/Node networking over HTTPS + HTTP/2 with real certificate verification (an ephemeral CA is installed in the collection host's trust store)
• V8 builtins profile: generated from an instrumented d8 built with Electron's exact V8 configuration; one x64 profile covers x64 + arm64 (block graphs are arch-independent, mirroring upstream)
• Upload: gated behind the pgo-upload GitHub environment (manual approval) and authenticated via OIDC — no long-lived credentials. Infra side: electron/infra#287 (merged)

Build-system changes

• build/args/pgo-instrument.gn / pgo-builtins-instrument.gn — instrumented build configurations
• New Chromium patch fixing profile-runtime resolution for instrumented builds on Linux arm32 and Windows cross-compiles (upstreamable; Chrome never builds these configurations)
• generate-chromedriver / generate-test-artifacts flags on the build segment (default true, no behavior change for existing callers) — instrumented builds skip artifacts only consumed by test jobs, saving ~10–20 min and ~700–950 MB per platform

What this PR does not do

Release builds do not consume these profiles yet — that's a follow-up PR (download profile + set pgo_data_path / v8_builtins_profiling_log_file). This PR only adds the generation side.

The pipeline was validated end-to-end on this branch's history: 13 full runs, with the final runs producing all 8 platform profiles + the builtins profile and uploading them to dev-cdn through the OIDC environment.

Checklist

• I have built and tested this change
• I have filled out the PR description
• I have reviewed and verified the changes
• npm test passes
• PR release notes describe the change in a way relevant to app developers

Release Notes

Notes: none