Create manifest.json

Fixes GoogleChrome#766

Create offscreen document, use WebRTC RTCDataChannel to send ArrayBuffer(s) from ServiceWorker to Web page.

Ideally this can be solved by programmatically setting WindowClient to arbitrary Web page, getting a reference to offscreen window, or setting opener on offscreen window for ability to use onmessage and postMessage(data, target, [data]) directly. 

RTCDataChannel send() does not transfer data. It does send ArrayBuffer(s) transferred from ServiceWorker to offscreen window to Web page window.

examples/transfer_offscreen_webrtc_web/manifest.json

@@ -0,0 +1,28 @@
+{
+  "name": "ServiceWorker => WebRTC => Web page",
+  "version": "1.0",
+  "description": "ServiceWorker => WebRTC => Web page",
+  "manifest_version": 3,
+  "background": {
+    "service_worker": "background.js",
+    "type": "module"
+  },
+  "host_permissions": [
+    "*://*.github.com/*"
+  ],
+  "permissions": [
+    "scripting",
+    "offscreen"
+  ],
+  "externally_connectable": {
+    "matches": [
+      "*://*.github.com/*"
+    ],
+    "ids": [
+      "*"
+    ]
+  },
+  "action": {
+    "default_title": "ServiceWorker => WebRTC => Web page"
+  }
+}