Add WebXR Chromium build and install scripts

Flathub Chromium can't enter immersive WebXR on Linux because upstream
only wires the OpenXR device on Windows. Document why, and add scripts to
cross-compile arm64 Chromium with the unmerged Linux OpenXR CLs and to
install, launch and check it on the Frame.

The first build is still running; immersive-vr support is unverified.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/build-chromium-xr.sh

@@ -0,0 +1,79 @@
+#!/bin/bash
+# Linux-side (x64 host): cross-compile arm64 Chromium with the Linux OpenXR CLs
+# (8441736 + 8132979, bug 506004811) so WebXR immersive-vr works on the Frame.
+# Needs ~90 GB free, no sudo. Takes hours; run it detached, e.g. on buildhost:
+#   scp scripts/build-chromium-xr.sh buildhost:chromium-xr/build.sh
+#   ssh buildhost 'cd ~/chromium-xr && tmux new -d -s chromium-xr "./build.sh > build.log 2>&1"'
+# Progress: ~/chromium-xr/stage. Output: ~/chromium-xr/chromium-xr-arm64.tar.xz,
+# which scripts/chromium-xr.sh install copies to the Frame.
+# Re-running resumes: existing checkout and out/XR are reused.
+set -euo pipefail
+W=~/chromium-xr
+cd "$W"
+stage(){ echo "$(date -Is) $*" | tee -a "$W/stage"; }
+# Returns non-zero below 12 GB free; set -e turns that into an exit at top level.
+guard(){ avail=$(df --output=avail -BG "$W" | tail -n 1 | tr -dc 0-9); if [ "$avail" -lt 12 ]; then stage "ABORT: only ${avail}G free for $W"; return 3; fi; }
+[ -d depot_tools ] || git clone -q https://chromium.googlesource.com/chromium/tools/depot_tools.git
+export PATH="$W/depot_tools:$PATH" DEPOT_TOOLS_UPDATE=1 DEPOT_TOOLS_METRICS=0
+CL_REF=refs/changes/79/8132979/44
+if [ ! -f .gclient ]; then
+  cat > .gclient <<'G'
+solutions = [{ "name": "src", "url": "https://chromium.googlesource.com/chromium/src.git",
+  "managed": False, "custom_deps": {}, "custom_vars": { "checkout_nacl": False } }]
+target_os = ["linux"]
+target_cpu = ["arm64"]
+G
+fi
+# Keyed on a real commit, so an interrupted first fetch is retried on re-run.
+if ! git -C src rev-parse -q --verify HEAD >/dev/null 2>&1; then
+  stage "clone src at $CL_REF"
+  mkdir -p src
+  [ -d src/.git ] || git -C src init -q
+  git -C src remote get-url origin >/dev/null 2>&1 || git -C src remote add origin https://chromium.googlesource.com/chromium/src.git
+  git -C src fetch -q --depth=1 origin "$CL_REF"
+  git -C src checkout -q FETCH_HEAD
+fi
+guard
+stage "src at $(git -C src log -1 --format='%h %s')"
+stage "gclient sync"
+gclient sync --nohooks --no-history -D --shallow --revision "src@$(git -C src rev-parse HEAD)" -j 8
+guard
+stage "runhooks"
+gclient runhooks
+src/build/linux/sysroot_scripts/install-sysroot.py --arch=arm64
+guard
+cd src
+mkdir -p out/XR
+cat > out/XR/args.gn <<'A'
+target_os = "linux"
+target_cpu = "arm64"
+is_debug = false
+is_official_build = false
+is_component_build = false
+dcheck_always_on = false
+symbol_level = 0
+blink_symbol_level = 0
+v8_symbol_level = 0
+proprietary_codecs = true
+ffmpeg_branding = "Chrome"
+use_remoteexec = false
+use_siso = true
+treat_warnings_as_errors = false
+A
+stage "gn gen"
+gn gen out/XR
+gn args out/XR --list=enable_openxr --short | tee -a "$W/stage"
+stage "build"
+( while sleep 600; do guard || { pkill -u "$(id -u)" -f "siso|ninja"; exit 3; }; done ) &
+GUARD=$!
+trap 'kill $GUARD 2>/dev/null || true' EXIT
+autoninja -C out/XR chrome chrome_sandbox chrome_crashpad_handler
+stage "package"
+cd out/XR
+files=(chrome chrome_sandbox chrome_crashpad_handler *.pak *.bin icudtl.dat locales)
+# GPU libraries aren't produced by every config; pack the ones that exist.
+for f in libEGL.so libGLESv2.so libvk_swiftshader.so libvulkan.so.1 vk_swiftshader_icd.json; do
+  [ -e "$f" ] && files+=("$f")
+done
+tar -cJf "$W/chromium-xr-arm64.tar.xz" "${files[@]}"
+stage "DONE $(ls -la $W/chromium-xr-arm64.tar.xz)"