Magnetometer: entire API is behind a flag in Chromium (again) (#25903)

Follow-up-to: #25897

Author: ddbeck

http/headers/Permissions-Policy.json

@@ -795,7 +795,14 @@
             ],
             "support": {
               "chrome": {
-                "version_added": "66"
+                "version_added": "66",
+                "flags": [
+                  {
+                    "type": "preference",
+                    "name": "#enable-generic-sensor-extra-classes",
+                    "value_to_set": "Enabled"
+                  }
+                ]
               },
               "chrome_android": "mirror",
               "edge": "mirror",
@@ -814,9 +821,7 @@
               },
               "safari_ios": "mirror",
               "samsunginternet_android": "mirror",
-              "webview_android": {
-                "version_added": false
-              },
+              "webview_android": "mirror",
               "webview_ios": "mirror"
             },
             "status": {