Electron Releases

Description of Change

Adds electron_xcache, a standalone host tool (//electron:electron_xcache) that compiles a JS file and emits a V8 code cache for the main process of a target Electron build of the same version — any OS/arch — given that build's V8 startup snapshot blob (or the binary embedding its Node startup snapshot: electron, electron.exe, Electron Framework).

• A code cache's only build-specific content is its header (version/flag hashes, RO-snapshot checksum) and ReadOnlyHeapRef offsets, all properties of the snapshot the consuming isolate was created from — so the tool creates its isolate from the target's blob, runs no JS beyond context setup, uses an inert external-reference table, and serializes.
• --mode script|function|module, --eager, --v8-flags / --extra-v8-flags (defaults to the main process's non-default set), --expect-flag-hash, --list to show the blobs found in a binary.
• Links only //v8, //v8:v8_libplatform, //third_party/zlib: one compile + one link on top of an existing build (~15 s incremental in a ThinLTO release dir).
• xcache.zip (binary + icudtl.dat + licenses; stripped on Linux official builds) is part of testing_build/release_build on every non-MAS build, staged with the other generated artifacts, and uploaded on publish as xcache-<version>-{darwin,linux,win32}-{x64,arm64}.zip. Any of them can target any platform; they differ only in which host they run on.
• spec/xcache-spec.ts (when the binary is present next to the build) checks the tool finds the running build's Node snapshot and that its script/function caches are accepted and execute correctly in the main process — guards the default flag set against drift.

Validated against 45.0.0-nightly.20260821: boots from all six desktop targets' embedded Node snapshots; output accepted and behaviourally identical to an uncached compile in the linux-x64, darwin-arm64 and darwin-x64 main processes (32/32 test scripts each, incl. lodash and typescript.js).

Checklist

• PR description included
• npm test passes

Release Notes

Notes: none