Adds a 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 release builds apply Chrome's published PGO profile, which
matches functions by symbol name and control-flow hash. Every function
that differs from Chrome - all of Node.js, patched Chromium code, V8
built with Node's flags, and the shell/ code - 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 because the Node integration changes their
codegen. Measured on Linux x64, an Electron-collected profile is worth
+9.5% on Speedometer 3.1 and +16% geomean across 22 app-operation
benchmarks (contextBridge calls +44-51%) versus Chrome's profile.

The pipeline:

- Instrumented builds (chrome_pgo_phase = 1) for linux x64/arm/arm64,
  win x64/x86/arm64, and macos x64/arm64, each collecting on hosts that
  can run its own binaries.
- Collection workloads: Speedometer 3 (x3), JetStream 2, and MotionMark
  (the same set Chromium's PGO recipe uses), plus Electron-specific
  phases browser benchmarks cannot cover: main-process Node.js
  (Buffer/crypto/fs/JSON), contextBridge and ipcRenderer.invoke
  marshaling, and renderer/Node networking over HTTPS + HTTP/2 with real
  certificate verification via an ephemeral CA installed in the
  collection host's trust store.
- A V8 builtins profile generated from an instrumented d8 built with
  Electron's exact V8 configuration; one x64 profile covers x64 and
  arm64 (builtin block graphs are arch-independent, mirroring upstream).
- Per-platform profraw sets merged with in-tree llvm-profdata tooling,
  then uploaded via OIDC through the pgo-upload GitHub environment
  (manual approval, no long-lived credentials).

Build-system changes:

- build/args/pgo-instrument.gn and pgo-builtins-instrument.gn
  instrumented build configurations (symbol_level = 0, no dSYMs -
  instrumented binaries are never shipped and do not fit on macOS
  runners with debug info).
- A Chromium patch fixing profile-runtime resolution for instrumented
  builds on Linux arm32 (clang package ships the armv7 triple; the
  driver resolves the arm triple) and Windows cross-compiles (objects
  embed the per-target runtime lib name; the Windows package ships the
  legacy layout). Chrome never builds these configurations; the patch is
  upstreamable.
- generate-chromedriver and generate-test-artifacts flags on the build
  segment and build-electron action (default true, no behavior change
  for existing callers) so instrumented builds skip artifacts only
  consumed by test jobs.

Release builds do not consume these profiles yet; that is a follow-up
change.

Co-authored-by: Sam Attard <sattard@anthropic.com>