Add MuteNotifications finch config to fieldtrial

This flag will be enabled on M88 Beta to launch the Mute Notifications
feature on Desktop platforms. This will prevent potentially sensitive
notification content from showing up on full-screen-share recordings.
Approved launch bug for Beta: crbug.com/1133886

Bug: 1133886
Change-Id: I4a549738a02d73883c87731e4cef1ce846a30de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577224Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834644}

testing/variations/fieldtrial_testing_config.json

@@ -4491,6 +4491,24 @@
             ]
         }
     ],
+    "MuteNotifications": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "MuteNotificationsDuringScreenShare"
+                    ]
+                }
+            ]
+        }
+    ],
     "MyChromeEverywhere": [
         {
             "platforms": [