New tabCapture.captureOffscreenTab API, initially for Presentation API 1UA mode

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This features file defines extension APIs implemented under src/chrome.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
@@ skipping 739 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "systemPrivate": {
     "dependencies": ["permission:systemPrivate"],
     "contexts": ["blessed_extension"]
   },
   "tabCapture": {
     "dependencies": ["permission:tabCapture"],
     "contexts": ["blessed_extension"]
   },
+  "tabCapturePrivate": {
+    "dependencies": ["permission:tabCapture"],
+    "contexts": ["blessed_extension"]
+  },
   "tabs": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"],
     "contexts": ["blessed_extension"]
   },
   "terminalPrivate": {
     "dependencies": ["permission:terminalPrivate"],
     "contexts": ["blessed_extension"]
   },
   "topSites": {
@@ skipping 75 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }