Prolong the expiry milestone for enable-webrtc-pipewire-capturer

The main reason for the flag is so called dialog hell, when you try to
share the screen on Wayland through PipeWire. To address it we've created
two changes, that are currently under review[0][1]. Once that's done the
flag could be removed.

[0] - https://webrtc-review.googlesource.com/c/src/+/160649
[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1942327

Bug: 682122
Change-Id: Ida77d4c8c16250201ae2b9f27e687325f4fcdd46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214885
Auto-Submit: Tomáš Popela <tomas.popela@gmail.com>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771838}

chrome/browser/flag-metadata.json

@@ -2386,7 +2386,7 @@
   {
     "name": "enable-webrtc-pipewire-capturer",
     "owners": [ "tomas.popela@gmail.com" ],
-    "expiry_milestone": 83
+    "expiry_milestone": 86
   },
   {
     "name": "enable-webrtc-remote-event-log",