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 the review is
done and code merged, then the testing needs to be figured out[2]. 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
[2] - https://bugs.chromium.org/p/chromium/issues/detail?id=1124347

Bug: 682122
Change-Id: Ifbe91fda3d58d5eeaace7aff438cc0c0e08dbde7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438054
Commit-Queue: Tomáš Popela <tomas.popela@gmail.com>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Tomáš Popela <tomas.popela@gmail.com>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811699}

chrome/browser/flag-metadata.json

@@ -2340,7 +2340,7 @@
   {
     "name": "enable-webrtc-pipewire-capturer",
     "owners": [ "tomas.popela@gmail.com" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "enable-webrtc-remote-event-log",