feat: [extensions] support extension.getBackgroundPage (#21951)

* feat: [extensions] support extension.getBackgroundPage

* cleanup

* how does c++

* tests

* test for runtime.getBackgroundPage too

shell/common/extensions/api/_api_features.json

@@ -9,6 +9,10 @@
     "extension_types": ["extension"],
     "contexts": ["blessed_extension"]
   },
+  "extension.getBackgroundPage": {
+    "contexts": ["blessed_extension"],
+    "disallow_for_service_workers": true
+  },
   "extension.getURL": {
     "contexts": ["blessed_extension", "unblessed_extension", "content_script"]
   }

shell/common/extensions/api/extension.json

@@ -12,6 +12,20 @@
     "properties": {
     },
     "functions": [
+      {
+        "name": "getBackgroundPage",
+        "nocompile": true,
+        "type": "function",
+        "description": "Returns the JavaScript 'window' object for the background page running inside the current extension. Returns null if the extension has no background page.",
+        "parameters": [],
+        "returns": {
+            "type": "object",
+            "optional": true,
+            "name": "backgroundPageGlobal",
+            "isInstanceOf": "Window",
+            "additionalProperties": { "type": "any" }
+         }
+      },
       {
         "name": "getURL",
         "deprecated": "Please use $(ref:runtime.getURL).",

shell/common/extensions/api/tabs.json

@@ -1,6 +1,7 @@
 [
   {
     "namespace": "tabs",
+    "description": "Use the <code>chrome.tabs</code> API to interact with the browser's tab system. You can use this API to create, modify, and rearrange tabs in the browser.",
     "functions": [
       {
         "name": "executeScript",